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What are clinical trials and how do they work?

A clinical trial is a research study designed to learn how our bodies respond to medicines or other treatments. Clinical trials:

  • Test new ways to help prevent, find, diagnose, or treat diseases
  • Help find out if new investigational treatments, or new uses for existing treatments, are safe and work well

Clinical trials are sometimes called clinical studies and clinical research.

Clinical trials happen in phases

Before health authority approval, any treatment (vaccine, medicine, medical device, or procedure) must go through 3 phases of clinical trials. Sometimes phase 4 trials are conducted after approval. Each phase tests the treatment’s safety, how well it works, amount (dose), and side effects. Participation in clinical trials is voluntary.

Phase 1

Investigational medication or vaccine is tested for safety on a relatively small group of 20 to 100 volunteers who are usually healthy, but not always.

The trial may happen in a doctor’s office or a hospital.

The goals of Phase 1 trials are to determine:

  • If the investigational medication or vaccine is safe
  • If there are any side effects
  • How the investigational medication is broken down by and discharged from the body
  • How much investigational medication is needed and how often
  • If vaccines cause the intended response from the body’s immune system

Phase 1 Trials can take six months to one year to complete.

Phase 2

Tests the medicine or vaccine in approximately 100 to 500 volunteers.

In the case of medicines, volunteers usually have the disease or condition the investigational medicine is designed to treat. In vaccine studies, the volunteers are usually healthy.

The goals of Phase 2 trials are to determine:

  • How well the investigational medication or vaccine works
  • If the investigational medication or vaccine is safe
  • If there are any side effects
  • How much of the investigational medication or vaccine is needed and how often

Phase 2 trials can take from six months to one year to complete.

Phase 3

Tests the medicine or vaccine in approximately 1,000 to 5,000 volunteers.

For medicines, volunteers have the disease or condition the medicine is designed to treat. In vaccine studies, the volunteers may be healthy or have diseases or conditions. Phase 3 trials may happen in a doctor’s office, a clinic, or a hospital.

Researchers closely monitor participants at regular intervals to:

  • Confirm that the investigational medication or vaccine is effective
  • Identify and monitor side effects
  • Compare the medicine or vaccine to commonly used treatments

Phase 3 trials can take one to four years to complete, depending on the disease, length of study, and number of volunteers.

Phase 4 (ongoing trials)

Thousands of people usually participate in ongoing trials.

Phase 4 clinical trials are conducted after the medicine or vaccine has been approved by the appropriate government and regulatory agencies and is being marketed.

Researchers continue to gather information about:

  • The medicine or vaccine and its safety, side effects, and effectiveness
  • Marketed products that are studied for new indications

Phase 4 trials are ongoing.

The importance of diverse participants

Age, race, gender, ethnicity, and many other factors play into how a person will react to an investigational medication treatment. To find out if a treatment is safe and works well for all people, researchers must have diverse participants in clinical trials. To read more about Merck’s commitment to diversity in clinical trials, visit the Diversity section.

Visit the Diversity page

Clinical trial participation

Taking part in a clinical trial is a process. We’ve outlined the steps you can expect before, during, and after being in a clinical trial.

Visit the Participation process page

What to consider before joining

Before you decide if you’ll join a clinical trial, learn as much as possible about it. You should feel comfortable asking the researchers all your questions about the trial, the care you can expect, and if there are costs. Explore a list of questions to ask the research team.

Visit the What to consider page

Still have questions?

Understanding clinical trials is important when making the decision to join one. The FAQ section has answers to commonly asked questions and a glossary of words to know.

Frequently asked questions
Glossary
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