Clinical trials can offer hope for many people and may help researchers find better treatments for others in the future.

What is a clinical trial?
A clinical trial is a research study in human volunteers that tests new ways to prevent, detect, diagnose or treat diseases. They help determine whether investigational vaccines, medicines, or new uses for existing medicines are safe and effective. Clinical trials are necessary to find potential medicines and vaccines that work to improve people’s health.

Clinical trials are also known as intervention studies, clinical studies or clinical research.

Phases of Clinical Trials

Clinical Phases

Phase 1: Phase 1 clinical trials test the medicine or vaccine in a small group of 20 to 100 volunteers who are usually healthy, but not always. The trial often takes place in a hospital. The goals are to determine:

  • If the medicine or vaccine is safe
  • If there are any side effects
  • How the medicine is broken down by and discharged from the body
  • How much medicine is needed and how often
  • For a vaccine, researchers see if it causes the desired response from the body’s immune system

Phase 1 trials can take from six months to one year to complete.

Phase 2: Phase 2 clinical trials test the vaccine or medicine 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 this phase are to determine:

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

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

Phase 3: Phase 3 clinical trials can test the medicine or vaccine in 1,000 to 5,000 patient 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 take place in hospitals, clinics or physician offices. Researchers closely monitor patients at regular intervals to:

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

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

Phase 4 or On-going 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 also are studied for new indications. Thousands of people usually participate in ongoing trials.

Careful Oversight

At Merck, our clinical trials are designed, conducted and monitored in accordance with local regulatory requirements, ethical principles and international standards that include:

Find a Study in US or Canada

Advanced Search    Search All Studies


Becoming knowledgeable and informed is a first step when considering participation in a clinical trial. These additional resources for patients and caregivers may help you. 

Discover some of the resources available for patients and caregivers. 


As you consider participating in a clinical trial, you may come across some new terms in your research.

View the glossary of terms.

Copyright © 2024 · Merck & Co., Inc., Rahway, NJ, USA, and its affiliates. All rights reserved. · This site is intended only for residents of the United States.
Cookie Preferences  Privacy  Terms of Use  U.S. Corporate Site