How Do Clinical Trials Work?

A clinical trial is a kind of research that involves treatment and people. Clinical trials help researchers answer questions such as:

  • Is a new treatment or medical device safe?
  • Does the new treatment do what it is supposed to do?
  • Is the new treatment better than a current treatment?
  • Which patients benefit the most from the new treatment?

Clinical trials are done in steps called phases. At each phase, researchers try to answer a different question. Each phase needs a different set of participants, so a participant joins only one trial at a time.

The four phases of clinical trials

Phase I: Is it safe?

  • This is the first time that a new treatment is tested with people so not many are included.
  • The main goal is to see if the treatment is safe.
  • People in Phase I studies can be healthy or have the disease.

Phase II: Does it work?

  • Once a treatment is safe, then a Phase II study is done to see if the treatment works.
  • Phase II studies usually have more participants than Phase I studies.

Phase III: Is it better than what we have now?

  • A Phase III study compares a new treatment with a current treatment to see which is safer and works better.
  • Phase III studies have large number of participants, sometimes thousands.
  • After a Phase III study shows that a treatment works and is safe, then the Food and Drug Administration (FDA) reviews it for approval.

Phase IV: What else can happen?

  • After FDA approval and the treatment is being used, researchers may do a Phase IV study.
  • Phase IV studies look at whether the treatment is useful for other conditions or has long-term problems.

The two main types of clinical studies

  • Observational studies are where the researcher takes no study-related medical action. Patients are treated based on normal clinical decisions if needed. People in these studies are monitored over a period of time and the study team collects and looks at data about the participants.

  • Interventional clinical trials are where the researcher takes action to prevent, understand better or treat a disease. They test the safety and effectiveness of a new drug or other treatment. Interventional clinical trials follow strict rules to protect patients.

For more information about clinical trials in general, see these pages at UCSF and the National Institutes of Health.