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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 safeSafe / SafetyAn assessment of the likelihood of causing an undesired effect and work well

Clinical trials are sometimes called clinical studies and clinical research.

Clinical trial volunteers play a vital role in the process an investigational medicine, vaccine, or medical device must go through before it can be approved for public use. By participating in clinical research, volunteers help researchers learn about how our bodies respond to investigational treatments and whether investigational treatments are effective. Learn how you can make a difference by participating in a clinical trial.

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 voluntaryVoluntaryTo choose to participate; to do something by choice; to participate because a person wants to; to participate without being forced or made to.

Phase 1

Investigational medication 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 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 1 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

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 our Diversity section.

Visit our 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 our 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 our list of questions to ask the research team.

Visit our What to consider page

Still have questions?

Our FAQ section has answers to commonly asked questions and a glossary of words to know.

Frequently asked questions

Glossary

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