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 or medical conditions
  • Help find out about the safety and efficacy of new investigational treatments, or new uses for existing treatments

Clinical trials are sometimes called clinical studies and clinical research.

Clinical trials happen in phases

Before approval for use, any treatment (vaccine, medicine, medical device, or procedure) must go through 3 phases of clinical trials. Sometimes phase 4 trials are conducted after approval. Participation in clinical trials is voluntary.


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. How well a medication works, its chance of causing side effects, and the type of side effects can differ by ethnic group. Including diverse participants in clinical trials helps researchers understand how the results will apply to the broader population. To read more about MSD’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?

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

Frequently asked questions