Clinical Trial Glossary: Key Terms and Definitions for Patients
Andrea Enguita 19 May 2026
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Andrea Enguita 19 May 2026
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Have you ever read clinical trial information and felt confused or overwhelmed by unfamiliar terms? You’re not alone. Clinical trials often involve technical language that can be difficult to understand, especially when you’re already dealing with important health decisions.
At myTomorrows, we work closely with patients and physicians every day, answering questions, providing guidance, and offering support as they explore clinical trials or, when appropriate, expanded access options. Over the years, we’ve supported more than 17,700 patients in more than 135 countries accessing investigational treatments through these pathways. This glossary is shaped by those real conversations and by our experience helping patients understand the clinical trial process.
Designed as a practical reference, this glossary is a resource you can return to whenever you need it. Whether you’re reading clinical trial information, reviewing consent documents, or preparing questions for your physician, it can help you better understand the terms that matter to you, so you can feel more confident and informed. The terms are listed alphabetically to make it easy to find what you’re looking for, whenever you need it.
An adverse event is any unwanted or unexpected health issue that happens during a clinical trial. It may or may not be caused by the clinical trial treatment.
This term matters because researchers record adverse events during the clinical trial to help understand how safe the treatment is and to identify possible risks.
You may come across this term in consent forms, trial information, or conversations about trial safety. For example, if a participant develops nausea, dizziness, or a rash during the trial, it may be recorded as an adverse event.
Adverse events can range from mild (such as headache or nausea) to serious (such as hospitalization or life-threatening illness).
Related terms: side effect, serious adverse event (SAE), safety monitoring.
A planned treatment path in a clinical trial. Each arm describes which treatment participants receive, such as an investigational treatment (experimental arm) or a placebo or standard care (control arm).
This term matters because clinical trials often compare results between different arms to understand how one treatment performs compared with another. In an open-label trial, knowing which arm you are in can help you understand what treatment you may receive.
You may come across this term in trial descriptions, consent forms, or when discussing how a clinical trial is designed with your doctor or the clinical trial team.
Related terms: control arm, placebo, randomization.
A test or procedure, such as a blood test, scan, or physical exam, used to collect information during a clinical trial.
This term matters because assessments help researchers monitor health, measure progress, and understand how a treatment is working.
Related terms: screening, clinical trial visit, endpoint
The standard medication or treatment a participant continues to receive while taking part in a clinical trial.
This term matters because it helps researchers understand how the clinical trial treatment works when added to the care a patient would normally receive.
Related terms: standard treatment (standard of care), investigational treatment, protocol.
Initial health measurements taken before the clinical trial treatment begins.
This term matters because these measurements give researchers a starting point to compare with later results during the trial.
Related terms: assessment, endpoint, screening
A type of clinical trial that studies one investigational treatment across different diseases or conditions that share a common biological feature, such as a specific genetic mutation.
This term matters because it can help researchers find out whether the same treatment may work for different groups of patients who have a similar biomarker.
Related terms: biomarker, eligibility criteria, clinical trial.
Factors that can unintentionally influence the results of a clinical trial and make them less accurate or reliable. Bias can happen, for example, when differences between participant groups or the way information is collected affects the clinical trial results.
This term matters because researchers use methods such as randomization and blinding to reduce bias and make clinical trial results more trustworthy.
Related terms: randomized clinical trial (RCT), blinding, control arm
An element in the body that can be measured and used as an indicator of health or disease.
This term matters because biomarkers can help researchers understand how a disease works, identify which patients may be eligible for a trial, and measure whether a treatment may be having an effect.
You may come across this term in genetic testing, lab reports, or eligibility criteria. For example, a trial may only include patients whose tumor has a specific genetic biomarker.
Related terms: eligibility criteria, surrogate endpoint, genetic mutation.
A method used to prevent participants, doctors, or clinical trial staff from knowing which treatment a participant is receiving, to reduce bias.
This term matters because blinding helps reduce bias and makes the clinical trial results more reliable. It helps ensure that expectations do not influence how symptoms, side effects, or outcomes are reported and interpreted.
You may come across this term in trial descriptions or consent forms.
Related terms: double-blind clinical trial, single-blind trial, placebo, bias
A type of research that tests an investigational treatment such as a drug, device, procedure or other intervention in people to find out if it is safe and effective for a specific condition.
This term matters because clinical trials are the main way new treatments are studied before they can become more widely available. Clinical trials help researchers understand benefits, risks, and how an investigational treatment compares with existing options. For example, a clinical trial may test whether a new cancer medicine works better than standard treatment.
You may come across this term when exploring treatment options or speaking with your healthcare team.
Related terms: investigational treatment, protocol, phase, interventional trial.
A unique identification number assigned to a clinical trial in a research registry, used to find and reference a specific trial.
This term matters because it helps patients, caregivers, and healthcare professionals make sure they are looking at the exact same trial, especially when trials have similar titles or involve similar treatments.
You may come across this term in trial registries, referral materials, emails from the clinical trial staff, or conversations about a specific trial.
For example, a doctor or Patient Navigator may share a clinical trial identifier so you can look up the full trial record online.
Related terms: clinical trial registry, protocol, recruitment status, clinical trial title
Healthcare professionals and researchers who support the trial, such as nurses, coordinators, and technicians.
This term matters because the clinical trial team is usually the main point of contact for participants during the trial.
Related terms: investigator, site, clinical trial visit
A scheduled appointment for trial-related tests, treatments, or check-ins, often at the trial sites.
This term matters because clinical trial visits are a key part of participation and can affect a patient’s time, travel, and daily routine. They are also how researchers collect information and monitor safety.
You may come across this term in the clinical trial schedule or consent form.
For example, a trial visit may include blood tests, scans, questionnaires, or receiving the clinical trial treatment.
Related terms: assessment, screening, follow-up, site.
A group of participants in a clinical trial who receive the same treatment or dose and are monitored together.
This term matters because researchers often organize participants into cohorts so they can study how a treatment affects each group.
Related terms: clinical trial group, treatment arm, dose.
A company hired by a trial sponsor to help plan, manage, and run a clinical trial, such as coordinating trial sites, collecting data, and monitoring safety.
This term matters because a CRO may support important parts of the clinical trial, such as coordinating sites, collecting data, or helping monitor how the trial is carried out.
Related terms: sponsor, site (clinical trial site), protocol.
Participants in a clinical trial who do not receive the investigational treatment. They may receive a placebo or standard care. Not all clinical trials include a control group.
This term matters because comparing the control arm with the investigational treatment arm helps researchers evaluate the treatment’s effects more clearly.
Related terms: treatment arm, placebo, standard treatment (standard of care).
An independent group of experts that reviews safety data during a trial.
This term matters because the committee helps protect participants’ safety by watching for important safety concerns or other issues during the trial.
Related terms: safety monitoring, serious adverse event (SAE), adverse event.
A type of clinical trial in which some or all trial activities involving the participant take place somewhere other than a single clinical trial site, such as at home or in local healthcare settings.
This term matters because it may make taking part in a trial easier by reducing the need to travel to the trial site for every visit.
Related terms: clinical trial visit, site (clinical trial site), clinical trial.
When a participant stops receiving the clinical trial treatment before the treatment period ends, but may continue with trial visits or follow-up.
This term matters because stopping the clinical trial treatment does not always mean leaving the trial, and researchers may still collect important follow-up information.
Related terms: follow-up, clinical trial visit, withdrawal
When a participant’s involvement in a clinical trial ends before it is completed. This may happen because the participant chooses to withdraw, or because the trial doctor stops participation for medical or other reasons.
This term matters because a participant may stop taking part in a clinical trial for different reasons, such as personal choice, side effects, or medical concerns.
Related terms: withdrawal, treatment discontinuation, follow-up.
The amount of treatment given at one time.
This term matters because the dose can affect how well a treatment works and what side effects may occur.
Related terms: treatment arm, phase 1 clinical trial, side effect.
How well a treatment works under controlled conditions, such as in a clinical trial.
Efficacy is usually measured in carefully designed studies where participants are closely monitored, and the treatment is used according to a strict protocol.
This term matters because how a treatment works in a clinical trial may not be exactly the same as how it works in everyday care, where people may have different health conditions, treatments, or levels of support.
Related terms: effectiveness, endpoint, clinical trial.
How well a treatment works in real-world medical practice.
Effectiveness reflects how a treatment performs when used by a wider group of people in everyday healthcare settings, outside the controlled environment of a clinical trial.
This term matters because a treatment may work differently in real life than it did in a clinical trial, where care is more closely monitored, and conditions are more controlled.
Related terms: efficacy, standard treatment (standard of care), endpoint.
Medical and personal requirements used to decide who can take part in a clinical trial.
These include inclusion criteria (the characteristics a participant needs to have) and exclusion criteria (the characteristics that may prevent participation).
This term matters because eligibility criteria help researchers decide whether a trial may be appropriate for a person, help protect participant safety, and support meaningful trial results.
You may come across this term when reviewing trial listings or speaking with the trial staff. For example, a trial may require a certain diagnosis, stage of disease, or previous treatment history.
Related terms: inclusion criteria, exclusion criteria, screening, pre-screening.
A specific result measured in a clinical trial to see whether a treatment is safe or effective, such as symptom improvement or survival.
Trials usually have a primary endpoint, which is the main result being measured, and secondary endpoints, which provide additional supporting information.
This term matters because endpoints show what the clinical trial is designed to measure and how researchers will judge the treatment results.
Related terms: efficacy, safety, protocol.
The process of officially joining a clinical trial after a person gives informed consent, completes screening, and is confirmed to be eligible.
This term matters because enrollment marks the point at which a person becomes a trial participant and begins taking part in trial-related procedures.
You may come across this term in trial updates or conversations with trial staff. For example, a person may complete pre-screening and screening but only be considered enrolled once all trial requirements are met.
Related terms: informed consent, screening, participant, eligibility criteria.
The regulatory agency that reviews and approves medicines for use in the European Union.
This term matters because the EMA reviews evidence from clinical trials to decide whether a treatment can be approved for patient use in the European Union.
Related terms: regulatory agency, clinical trial, investigational treatment, Food and Drug Administration (FDA).
The specific characteristics or conditions that prevent a person from being eligible to join a clinical trial.
These may include factors such as certain medical conditions, previous treatments, unexpected test results, or other risks that could affect safety or the clinical trial results.
This term matters because exclusion criteria help researchers decide whether a trial may be appropriate for a person and help support participant safety and meaningful clinical trial results.
Related terms: eligibility criteria, inclusion criteria, screening, pre-screening.
A way for patients with serious or life-threatening conditions to receive an investigational drug or treatment outside of a clinical trial when no other satisfactory treatment options are available and after the assessment by their doctor.
This term matters because it may be another way to get access to an investigational treatment when joining a clinical trial is not possible or not appropriate.
You may come across this term when exploring investigational treatment options with your doctor, especially if there is no suitable clinical trial available. For example, a patient who cannot join a clinical trial may, in some cases, be considered for expanded access if their doctor and the company developing the treatment agree, and local requirements are met.
Related terms: investigational treatment, clinical trial, treating physician.
For a more detailed explanation of expanded access and related terms, see “Understanding expanded access, compassionate use, and similar terms.”
The monitoring of a participant’s health and outcomes after the main treatment phase of the clinical trial has ended.
This term matters because follow-up helps researchers understand longer-term safety, side effects, and treatment outcomes, even after a participant stops receiving the trial treatment.
You may come across this term in clinical trial schedules or consent forms. For example, a participant may return for scans or check-ins several months after treatment has finished.
Related terms: clinical trial visit, safety monitoring, discontinuation, endpoint.
The regulatory agency that reviews and approves medicines for use in the United States.
This term matters because the FDA reviews evidence from clinical trials to decide whether a treatment can be approved for patient use in the United States.
Related terms: regulatory agency, clinical trial, investigational treatment, European Medicines Agency (EMA)
The specific characteristics a person must have to be eligible to join a clinical trial.
These may include factors such as age, diagnosis, stage of disease, previous treatment history, or certain test results.
This term matters because inclusion criteria help researchers decide whether a trial may be appropriate for a person and help support participant safety and meaningful clinical trial results.
Related terms: eligibility criteria, exclusion criteria, screening, pre-screening.
The disease, condition, or situation for which a treatment or test is intended to be used.
This term matters because it helps explain what a treatment is meant to treat and whether it may be relevant to a person’s condition.
Related terms: standard treatment (standard of care), investigational treatment, eligibility criteria.
The process of explaining a clinical trial, including its risks and benefits, so that individuals can make an informed decision about taking part.
If they agree, participants must sign a consent form before joining.
This term matters because informed consent helps protect patient autonomy and ensures that participation is voluntary and based on an informed decision. It is a key ethical part of clinical research.
You may come across this term before screening or enrollment. For example, a participant is usually asked to review and sign a consent form before any trial-specific procedures begin.
Related terms: consent form, enrollment, eligibility criteria, Institutional Review Board (IRB) / Ethics Committee
An independent, multidisciplinary group that reviews and monitors clinical trials to help protect the rights, safety, and well-being of participants.
This term matters because this group reviews the trial before it begins and may continue to oversee it to help make sure participants are treated ethically and safely.
Related terms: informed consent, participant, safety monitoring.
A research in which participants receive an investigational treatment such as a drug, device, or procedure being studied to evaluate its effects on health. Interventional studies are commonly called clinical trials.
This term matters because it distinguishes studies that actively test an intervention from studies that only observe what happens without assigning treatment.
You may come across this term in research registries or trial descriptions. For example, a trial testing a new investigational medicine in people with a specific disease is an interventional trial.
Related terms: clinical trial, observational trial, investigational treatment, protocol
A drug, device, procedure or other intervention that is being studied to treat a specific condition and has not yet been approved for general use.
This term matters because investigational treatments are the central focus of many clinical trials. Understanding this term helps patients recognize that the treatment is still being studied and may not yet be proven safe or effective.
You may come across this term in trial descriptions, consent forms, or discussions with the clinical trial staff.
Related terms: clinical trial, phase, placebo, standard treatment (standard of care).
The doctor or scientist responsible for leading and overseeing the clinical trial at a trial site.
This term matters because the investigator is responsible for how the clinical trial is carried out at that site and for the care and safety of participants.
Related terms: site (clinical trial site), Clinical trial team (clinical trial staff), protocol.
An assessment by the clinical trial doctor or research team to determine whether a person may be suitable for a clinical trial.
This term matters because the clinical trial team may review medical records, test results, and treatment history to decide whether participation seems appropriate.
Related terms: eligibility criteria, screening, pre-screening.
An observational trial is a type of research in which researchers collect health information from participants over time without assigning a treatment. It is not a clinical trial, because participants do not receive an investigational treatment.
This term matters because observational studies are different from clinical trials. They help researchers understand health and disease over time without assigning a trial treatment.
You may come across this term in trial registries or research materials. For example, a trial that tracks symptoms and test results in people living with a condition over time may be observational.
Related terms: interventional trial, clinical trial, registry, patient-reported outcomes (PROs)
A clinical trial in which both participants and researchers know which treatment is being given.
This term matters because knowing which treatment is being given can affect how results are reported and interpreted, especially for symptoms or side effects.
You may come across this term in trial descriptions, consent forms, or discussions about how a trial is designed.
Related terms: blinding, placebo, bias.
Clinical trials that offer payment or compensation to participants for their time, travel, or inconvenience related to taking part in the clinical trial.
This term matters because payment may help reduce some of the practical burden of taking part in a trial.
Related terms: reimbursement, clinical trial visit, participant.
A person who has enrolled in a clinical trial and takes part in trial-related procedures. Participation is voluntary, and people may choose to leave the clinical trial at any time.
This term matters because it refers to someone who is officially taking part in a clinical trial, not just asking about it or being screened for it.
Related terms: enrollment, informed consent, screening.
A professional with a medical background who helps patients discover and potentially access clinical trials. They serve as a consistent point of contact and guide patients through the process.
This term matters because the clinical trial process can be complex and overwhelming. A Patient Navigator can help explain terms, answer practical questions, and guide patients through the steps involved in exploring options.
You may come across this term when seeking support with trial discovery or understanding eligibility. For example, a Patient Navigator may help a patient review possible studies and understand what information is needed for pre-screening.
Related terms: eligibility criteria, pre-screening, clinical trial, expanded access
Information participants provide directly about their own health, symptoms, or well-being as part of a clinical trial.
This term matters because it shows that a patient’s own experience is an important part of how a treatment is evaluated.
Related terms: endpoint, assessment, quality of life.
The different stages of clinical testing for a new treatment, usually numbered Phase 0 through Phase 4. The design of each phase may vary depending on the disease being studied, the existing standard treatment (standard of care), and ethical considerations. For example, some Phase 2 studies may include a placebo for comparison. However, the phases are generally defined as follows (see below).
For a more detailed explanation of each clinical trial phase, see the blog “Introducing clinical trials: phase 1–4 explained”
This term matters because the phase of a trial can help you understand how much is already known about the treatment and what the trial is mainly designed to find out.
You may come across this term in trial listings or discussions with trial staff. For example, a Phase 1 trial usually focuses on safety and dose, while a Phase 3 trial often compares a treatment with standard treatment or placebo.
Related terms: phase 0 clinical trial, phase 1 clinical trial, phase 2 clinical trial, phase 3 clinical trial, investigational treatment.
An early, small clinical trial using very low doses of a treatment to see how it behaves in the human body before Phase 1 begins.
This term matters because it helps researchers learn early information about a treatment before larger studies begin.
Related terms: phase, phase 1 clinical trial, investigational treatment.
An early clinical trial that tests a new treatment in a small number of people to check safety, find the right dose, and understand side effects.
This term matters because Phase 1 trials help researchers understand safety, side effects, and dose before moving into later studies that look more closely at how well a treatment works.
Related terms: phase, dose, side effect.
A clinical trial conducted in people with a condition that focuses on whether a treatment works, while continuing to monitor safety and side effects.
This term matters because results from Phase 2 trials can help researchers decide whether the treatment should move into larger studies.
Related terms: phase, efficacy, phase 3 clinical trial
A large clinical trial that compares an investigational treatment to standard treatment or a placebo. The goal is to confirm efficacy and better understand risks before approval.
This term matters because Phase 3 trials often provide the main evidence used to decide whether a treatment should be approved for wider patient use.
Related terms: phase, standard treatment (standard of care), placebo, investigational treatment.
Research done after a treatment is approved to monitor long-term safety, side effects, and how well it works in everyday use.
This term matters because these studies help researchers learn more about how a treatment performs after it becomes available to a wider group of patients.
Related terms: phase, effectiveness, side effect.
A substance or treatment that looks like the investigational treatment but has no active medicine. It is used for comparison with the investigational treatment in a clinical trial. For example, in some randomized trials, one group may receive the investigational treatment and another may receive a placebo.
This term matters because a placebo can help researchers compare results and understand whether the effects seen in a clinical trial are likely due to the investigational treatment. They are one of the tools used to make trial results more reliable.
You may come across this term in consent forms, trial descriptions, or conversations about how a clinical trial is designed.
Related terms: randomization, blinding, control arm, standard treatment (standard of care)
A type of clinical trial that studies how a treatment works in everyday medical care, rather than under highly controlled research conditions.
This term matters because the results may be more relevant to everyday care, since these studies often include a broader range of patients and more usual healthcare settings.
Related terms: effectiveness, clinical trial, standard treatment (standard of care)
Laboratory studies (such as cell testing) and animal research done before a treatment is tested in people, to learn about safety and how it may work.
This term matters because preclinical studies are an important early step in understanding whether a treatment may be safe enough to move into human testing.
Related terms: investigational treatment, phase 1 clinical trial, safety
An initial review done before the formal onsite screening process to see whether a person may be a potential match for a clinical trial. Pre-screening often involves reviewing basic information, such as diagnosis, age, previous treatments, or medical history, to determine whether it is worth proceeding to full screening.
This term matters because pre-screening can save time and reduce burden by identifying early whether a clinical trial may be relevant before more detailed screening procedures take place.
You may come across this term when first speaking with a trial site or support service. For example, basic details such as diagnosis, age, and treatment history may be reviewed during pre-screening.
Related terms: screening, eligibility criteria, enrollment, investigator review
The detailed plan for how a clinical trial will be carried out, including who can participate, what tests are done, and how long the trial lasts.
The protocol may be updated if changes are needed (protocol amendment).
This term matters because the protocol guides every part of the clinical trial and helps ensure that all participants are treated consistently and safely. It also explains how researchers will measure results.
You may come across this term in trial summaries, consent materials, or discussions with clinical trial staff. For example, the protocol may specify how often clinical trial visits happen and which assessments are required.
Related terms: endpoint, eligibility criteria, protocol amendment.
Assigning participants to different clinical trial arms by chance to avoid bias. Depending on the trial, this may mean being assigned to the investigational treatment, standard treatment, or a placebo.
This term matters because randomization helps create groups that can be compared more fairly. This helps researchers understand whether differences in results are likely due to the treatment being studied, rather than other factors.
You may come across this term in trial descriptions, consent forms, or conversations about how a clinical trial is designed.
Related terms: randomized clinical trial (RCT), placebo, blinding, treatment arm
A type of clinical trial in which participants are assigned by chance to different treatment arms to reduce bias.
This term matters because randomization helps researchers compare treatments more fairly, for example by comparing an investigational treatment with standard treatment or a placebo.
Related terms: randomization, placebo, control arm
The current stage of a clinical trial, such as whether it is recruiting, not yet recruiting, active but not recruiting, completed, or stopped.
This term matters because it helps patients understand whether a trial may still be open to new participants.
Related terms: enrollment, clinical trial identifier, trial capacity.
A collection of health information about people with a specific condition, often used to support research or future clinical trials.
This term matters because registries can help researchers understand patterns in a condition and may support future clinical studies.
Related terms: observational trial, clinical trial, Patient-reported outcomes (PROs)
A government organization that reviews clinical trial data and decides whether a treatment can be approved for patient use.
This term matters because regulatory agencies play an important role in deciding whether a treatment can become more widely available.
Related terms: European Medicines Agency (EMA), Food and Drug Administration (FDA), investigational treatment.
Payment or coverage for trial-related expenses, such as travel, meals, or lodging.
This term matters because taking part in a clinical trial may involve practical costs, and reimbursement can affect whether participation is manageable for a patient and their family.
You may come across this term when reviewing clinical trial information or asking the trial site about costs. For example, a clinical trial may reimburse travel expenses for visits to the hospital or clinic.
Related terms: paid clinical trials, site visit, site, trial-related expenses.
Ongoing checks to identify side effects or risks during a trial.
This term matters because it helps protect participants throughout the clinical trial by making sure their safety is checked regularly.
Related terms: adverse event, serious adverse event (SAE), Data monitoring committee (DMC)
Medical tests and evaluations done to determine whether someone is eligible and can join a clinical trial.
This term matters because screening helps the trial team decide whether the trial may be appropriate and safe for a person before enrollment. Passing pre-screening does not guarantee passing screening.
You may come across this term when the clinical trial team starts formally checking whether a trial may be a suitable option for you.
Related terms: pre-screening, eligibility criteria, enrollment, screen failure
When a person goes through screening for a clinical trial but is not enrolled because they do not meet all eligibility criteria or another issue prevents participation.
This term matters because it does not necessarily mean something is wrong. It simply means the trial was not considered the right match at that time.
Related terms: screening, eligibility criteria, enrollment
An adverse event that results in serious health consequences, such as death, a life-threatening situation, hospitalization or significant disability.
This term matters because serious adverse events require close attention and reporting during a clinical trial. They are an important part of how safety is monitored and assessed.
You may come across this term in trial documents, discussions about possible risks, or consent forms.
Related terms: adverse event, side effect, safety monitoring, safety reporting
A type of adverse event (unwanted effect) believed to be related to the treatment.
This term matters because side effects are an important part of understanding how safe a treatment may be.
Related terms: adverse event, serious adverse event (SAE), safety monitoring
The location, usually a hospital or clinic, where clinical trial visits, treatments, and procedures take place.
This term matters because the clinical trial site is where participants interact with the trial team, complete assessments, and receive treatment. Location can also affect access, travel needs, and scheduling.
You may come across this term when searching for trials or discussing logistics. For example, a trial may be open in several trial sites across different cities or countries.
Related terms: clinical trial visit, investigator, screening, trial capacity
The organization (such as a pharmaceutical company, university, or foundation) that funds and manages a clinical trial.
This term matters because the sponsor plays a major role in designing the clinical trial, overseeing operations, and supporting data collection, safety review, and regulatory steps.
You may come across this term in trial registries or consent documents. For example, a pharmaceutical company may sponsor a trial testing a new investigational treatment.
Related terms: Contract Research Organization (CRO), protocol, regulatory agency, investigational treatment
The treatment most commonly used and accepted as appropriate for a specific condition.
This term matters because standard treatment is often what a new investigational treatment is compared against in some clinical trials. It also reflects the care a patient would usually receive outside the clinical trial.
You may come across this term in trial descriptions or treatment discussions. For example, a trial may compare a new treatment plus standard treatment against standard treatment alone.
Related terms: placebo, control arm, investigational treatment, phase 3 clinical trial
A biological measurement (such as a blood test result or imaging finding) that serves as an indirect indicator of clinical benefit. It is used as a surrogate endpoint in a clinical trial to help estimate whether a treatment is working, without waiting for longer-term outcomes.
This term matters because it can help researchers estimate treatment effects earlier, without waiting for longer-term clinical outcomes. Some trials rely on these measurements to make earlier decisions about whether a treatment should continue being studied.
Related terms: surrogate endpoint, biomarker, endpoint.
An indicator used in clinical trials to provide an early estimate to whether a treatment may be working, instead of a long-term outcome (such as living longer).
This term matters because it helps explain why a treatment may appear to be working in a trial even before its full clinical benefit is known.
Related terms: endpoint, surrogate biomarker, biomarker
The number of participants a clinical trial site is able to enroll at a given time.
Trial capacity may be limited by the clinical trial design, the number of available places, staffing, or site resources.
This term matters because a trial may not be able to accept more participants once all available places have been filled.
Related terms: recruitment status, enrollment, clinical trial site.
A planned period of time during which a participant stops taking a previous treatment before starting the clinical trial treatment.
This term matters because it helps reduce the chance that an earlier treatment could affect the clinical trial results or interact with the treatment being studied.
Related terms: investigational treatment, eligibility criteria, protocol
When a participant chooses to leave a clinical trial at any time, for any reason.
This term matters because participation in a clinical trial is voluntary. Understanding withdrawal helps patients know that they have the right to stop taking part if they choose.
You may come across this term in informed consent materials, trial documents, or conversations with the clinical trial team.
For example, a participant may decide to withdraw because of side effects, personal reasons, or a change in treatment plans.
Related terms: informed consent, clinical trial discontinuation, participant, follow-up
Eligibility means the rules used to decide who can join a clinical trial. These rules include inclusion criteria (what someone must have) and exclusion criteria (what may prevent someone from joining). They help ensure the trial is safe and appropriate for people taking part, and that the trial results are reliable and meaningful.If you are unsure whether you may qualify, the clinical trial team can explain how the criteria apply to your situation..
The clinical trial team at the trial location usually decides whether you can join. The decision is based on the trial’s eligibility criteria, your medical information, test results, and overall health. In some cases, your doctor may also help by sharing information or making a referral. The final decision is based on whether the clinical trial is considered appropriate and safe for you. .
You may not have been eligible because the clinical trial was not the right match for your medical situation at that time. Each clinical trial has specific rules about things like diagnosis, stage of disease, past treatments, lab results, and overall health. Not qualifying does not mean something is wrong or that you will not qualify for another clinical trial. It simply means that this particular clinical trial was not considered suitable.
If pre-screening suggests you may be a match, the next step is usually formal screening at the clinical trial location. This may include reviewing your medical records, signing informed consent forms, and completing tests such as blood work, scans, or physical exams. Pre-screening does not mean you are already accepted into the clinical trial. The clinical trial team still needs to confirm that you meet all eligibility criteria.
Yes, in many cases you can explore or be considered for more than one clinical trial at the same time. This is often possible during the search or pre-screening stage, when you are still learning which options may be the best fit. However, people are usually not allowed to take part in more than one interventional clinical trial at the same time. The clinical trial team can explain whether one trial may affect your eligibility for another.
If a clinical trial is full, it means the trial or a specific location has already enrolled the number of participants it planned to include. In that case, no more people can join unless a place opens up or enrollment expands. Sometimes another location may still be recruiting, even if one location is full. Trial availability can also change over time, so it may be worth asking whether there are other participating sites, similar studies, or future trial opportunities that may be relevant to your situation.
A clinical trial may help some people, but it does not guarantee improvement. Clinical trials are designed to study how safe a treatment is and how well it works, so outcomes can vary from person to person. Some participants may benefit, some may not, and side effects are also possible. It is a good idea to talk with your doctor or clinical trial team about the possible risks and benefits before deciding.
The costs can vary depending on the clinical trial and where it takes place. In many cases, the sponsor covers the investigational treatment and any tests or procedures required only for the clinical trial. But there may still be other costs, such as routine care, travel, meals, lodging, or time away from work. Before enrolling, you can ask the clinical trial team what is covered, what may be billed to insurance, and whether any reimbursement is available.
Your doctor may be focused first on standard treatment options that are already available and well understood. They may also not know about every clinical trial recruiting for your condition, especially if trials are being run at other hospitals, in other regions, or in other countries. In some cases, there may not be a suitable clinical trial for your diagnosis or stage of disease..
If you are interested in learning about clinical trials, you can ask your doctor about whether a clinical trial might be appropriate for you and what options may be available.
myTomorrows helps patients and their treating physicians discover, understand, and access clinical trial options or, when appropriate, expanded access programs. Our Patient Navigators can help explain the process, answer practical questions, and support you as you explore possible next steps.
Our Patient Navigators are here to help you learn more about clinical trials and how they may be relevant to your specific situation.
If you’d like to speak with a Patient Navigator, you can book a (free) call at a time that suits you.
Getting started with clinical trials:
Clinical trials by disease
Understanding expanded access and compassionate use
About the author
Medical Content Writer at myTomorrows
Dr. Andrea Enguita Marruedo holds a master’s in Genetics and Cell Biology from the Autonomous University of Madrid and a PhD in Developmental Biology from the Erasmus Medical Centre in Rotterdam. She began her career in medical writing after completing her doctoral studies and has covered a wide range of therapeutic areas, including neuromuscular and neurodegenerative disorders, cancer, and diabetes.
Andrea specializes in translating complex biomedical topics into clear, accessible content for patients and healthcare professionals. With a strong research background and a passion for science communication, she is committed to delivering accurate, well-referenced content that supports greater awareness and understanding of medical topics among patients, caregivers, and clinicians.
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