Clinical trials

Learn more about clinical trials, why they matter, and how to participate at NIH Clinical Trials and You.

For more information about clinical trials taking place, visit

CISCRP (Center for Information & Study on Clinical Research Participation) is a nonprofit organization that provides information on clinical trials for potential volunteers and features a searchable database of ongoing studies. It also offers educational resources and analysis tools for researchers and clinicians.

Clinical trials are studies with human participants. In a clinical trial, scientists want to find out if a solution to a medical problem works. The goal of a clinical trial is to test new treatments to make sure they are effective and safe. All new treatments must be approved by the U.S. Food and Drug Administration (FDA).

Before a clinical trial happens, many other studies – called preclinical studies – take place. These studies are done to figure out whether the treatment is safe to be tested in humans. After a treatment passes through preclinical studies, a clinical trial in people can start.

It takes a team of many people to do a clinical trial. Doctors, nurses, social workers, and other healthcare professionals work together in the clinic to do these studies. Clinical trials usually also have a company sponsor. The company pays for the study, designs the study, analyzes the data, and reports the results. There are many rules and regulations that must be followed when doing a clinical trial. The clinical team, industry partner, and the FDA work closely together as a treatment, or drug, progresses through clinical trials.

Kinds of Clinical Trials

There are four kinds of clinical trials. Each kind has a different purpose.

  • Treatment trials test new treatments or new ways to do a medical procedure like surgery.
  • Prevention trials look for better ways to prevent disease.
  • Diagnostic and Screening trials focus on better ways to find out if you have a disease or condition.
  • Quality of Life trials try to improve the lives of people with life-long diseases.

The Stages of Clinical Trials

Clinical trials are done in stages called phases. Each phase tries to answer a different question (see the questions below). The FDA tracks each phase of the clinical trial. Certain benchmarks must be met before moving to the next phase.

The phases of a clinical trial that happen before a new treatment is approved are:

Phase I: Is it safe?

In phase I, a small group of people (20-80) participate. Researchers test how much of a medicine, or drug, is safe for people to take. Researchers also look at what side effects the drug causes. Phase I trials usually test the treatment in healthy people. There are some exceptions, such as cancer and HIV.

Phase II (2): Does it work?

Phase II involves a larger group of people (100-300). In this phase researchers measure how well the drug works. They also study how much of the drug to give people for it to work best. 

Phase III (3): Is it safe and does it work?

Phase III includes 1,000-3,000 people. Scientists keep testing how well the drug works. They may also compare the new drug to other drugs to see if it works better.

Phase IV (4): Continued studies

Phase IV trials begin after the FDA approves a drug. They are also known as post-marketing studies. These studies give more information about the drug’s risks and benefits and the best ways to use it.

Other things to think about

I'm getting the treatment...or not?

Most clinical trials include a control group. The control group often receives the current treatment standard or a placebo (no treatment). Researchers compare data from the control group with data from the group getting the new treatment. This is now researchers know if the new treatment is working and if it is better than the current treatment.

If you participate in a clinical trial, you cannot be sure that you will receive the new treatment. You may be part of the control group. There is also no guarantee that the new treatment will make you better. It may make you better, it may have no effect, or it may make your symptoms worse. You may also have side effects from the new treatment. While most side effects are mild, it is possible the new treatment may make you sick. The clinical team will watch to see if you have any side effects.

Treatment or no treatment: Who knows?

The researchers usually do not know if you are receiving the drug or the placebo. This is called blinding. Blinding makes sure that the researchers or participants do not change the results of the trial by knowing who is receiving the new drug.

Why and when might a trial be stopped?

The FDA may stop a trial if the group getting the treatment is doing very well. In this case all participants should be offered the new treatment. The FDA also may stop a clinical trial if it is not safe. Trials can be stopped at any phase.

Know before you sign up

Before participating in a clinical trial, you will be part of the informed consent process. It is important that no one makes you participate. It is also important that you understand what you are agreeing to. The researchers will talk about the study with you, including risks and benefits, and answer all of your questions.

Children and clinical trials