People count on us to make medicines and vaccines that have well documented safety and effectiveness profiles and offer value. Clinical trials (also known as clinical studies) are a critical step in this process.

Each clinical trial is designed to answer certain research questions. They follow strict, predefined protocols to ensure safe and accurate results. Each phase has a different purpose in the development of a medicine or vaccine:

• Phase I: Researchers test an experimental drug or treatment in a small group of people (20-80) for the first time to evaluate its safety, determine an appropriate dosage, trace what happens to the compound in the body, and begin to identify side effects.
• Phase II: The drug or treatment being studied is given to a larger group of people with the disease (100-300) to obtain preliminary evidence of an effect and to further evaluate its safety. If there is evidence of an effect and the risks are considered acceptable, the drug may move to the next stage.
• Phase III: This phase of trial tests a drug candidate in a larger number of people with the disease (1,000-3,000), further testing its effectiveness and monitoring for side effects. In some cases this will be compared to a treatment standard, if one is already available.
• Phase IV: After a drug or treatment has been approved by the appropriate government and regulatory agencies and is being marketed, we may study its safety and effectiveness over a longer period of time and in a larger number of people. We may also continue to study some of our marketed products for new indications. Thousands of people usually participate in Phase IV trials.

At MSD, our clinical trials are designed, conducted and monitored in adherence to the same standards, whether they take place in the United States or elsewhere around the world. In conducting clinical trials, we also adhere to the guidelines of The International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use.

Results of MSD-sponsored Clinical Trials

​We are committed to the timely registration of clinical trials in patients and disclosure of those clinical trial results regardless of their outcome. We seek to provide balanced and accurate information about our studies, and have strict guidelines in place to guide publication. We also disclose medically important safety data from other studies. In addition to publishing our clinical trial results in peer-reviewed medical journals, we post study results of our marketed products on

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