What Is a Clinical Trial? Phases Explained
A clinical trial is a research study that tests how well a new medical approach works in people. The approach might be a drug, a procedure, a screening method or a way of managing symptoms. Trials are how doctors learn whether something is safe and whether it helps, and the NCI notes that people are living longer thanks to past cancer trials. This page explains in general terms how trials are organised and what the phases mean. It is not medical advice, and anyone considering a trial should talk with their own medical team.
What clinical trials are for
The US National Cancer Institute (NCI) describes clinical trials as research studies that test how well new medical approaches work in people. In cancer, they look at ways to find, prevent and treat the disease, and at ways to manage the side effects of illness and treatment.
Several types exist:
- Treatment trials test new treatments or new ways of using existing ones. They are the most common type.
- Prevention trials look at ways to lower the chance of developing cancer, often in people who are at higher risk.
- Screening trials test methods for finding cancer before symptoms appear.
- Supportive and palliative care trials study ways to improve quality of life, especially for people with side effects from the illness or its treatment.
The phases of a trial
New treatments usually move through stages, each designed to answer a different question. The figures below come from the NCI’s description of the phases.
| Phase | Main question | Typical size |
|---|---|---|
| Phase 1 | Is it safe, what are the side effects, and what dose is tolerated? | Around 15 to 30 people |
| Phase 2 | Does it appear to work against the disease, while safety continues to be studied? | 50 to 100 people |
| Phase 3 | How does it compare with the current standard treatment? | From 100 to several thousand people |
| Phase 4 | What are the long-term safety and effectiveness once it is approved and in wider use? | Large, diverse groups |
Sizes vary, and the table is a general outline, not a rule for any individual study. The NCI explains each phase on its page about the phases of clinical trials.
How a trial is designed
Randomisation. In many phase 3 studies, participants are assigned to groups by chance alone. The NCI notes that this helps prevent bias. One group typically receives the new approach, and a control group receives the standard treatment.
Placebos. A placebo is an inactive substitute. The NCI says placebos are rarely used in cancer treatment trials, and any use would be explained in the consent form.
Eligibility criteria. Each trial lists requirements for who can take part. Age is a common criterion, and others can include the type of illness and previous treatments. These rules exist to keep participants safe and to make the results meaningful, though they also mean not everyone who is interested can join.
Informed consent. Trials use a consent process in which the study team explains the purpose, the procedures and the possible risks and benefits, and the person can ask questions. Taking part is voluntary.
Why rare cancers are hard to study
When a disease is rare, there are fewer patients to enrol, and it can take a long time to collect enough results. This is one reason there are relatively few trials for very rare cancers such as alveolar soft part sarcoma, and why studies are often small and spread across a few specialist centres.
News stories sometimes discuss who is allowed into a trial and how patients can obtain treatments outside a study. This site keeps a short summary of news coverage from 2014 on those themes, and it is a reminder that eligibility rules are a real part of how research works.
Questions people ask before considering a trial
- What is the goal of the study?
- What happens at each visit, and how long does it last?
- What are the known risks and side effects?
- Which costs, if any, are covered? Arrangements vary by country and trial.
- What happens if I want to leave?
- Who can I contact with questions?
A person’s treating doctor is the best source for judging whether a particular trial is relevant. The NCI advises talking with your doctor about clinical trials when discussing treatment options, and offers a search tool for finding trials.
Frequently asked questions
Are clinical trials only for people who have run out of options?
No. Trials can involve people at different stages of illness, as well as people with no illness at all, such as in prevention and screening studies. The purpose depends on the type of trial.
Does being in a trial mean receiving a placebo?
Not usually in cancer treatment trials. The NCI says placebos are rarely used there, and if a placebo is part of a study, it must be explained in the consent form.
Can someone leave a trial after joining?
Participation is voluntary. The consent process should explain what leaving involves, so ask the study team if anything is unclear.
Is a trial the same as a treatment?
No. A trial is a research study. It can include a treatment, but its main goal is to learn whether the approach is safe and effective.
Where can I find reliable information?
National cancer organisations, such as the NCI, publish plain-language guides and trial search tools. Medical professionals can explain how a specific study applies to an individual.
Understanding the phases and the language makes trial information easier to follow. Always use official sources and professional advice for personal decisions.