Phases of Clinical Trials

Every prescription medicine available in the United States must be evaluated in a series of clinical trials. Each of these "phases" is followed closely by a regulatory review and approval process. Sanofi conducts clinical trials in every phase.

Phase 1

In Phase 1 clinical trials, researchers test an investigational drug or vaccine for the first time on a small group of healthy volunteers (generally less than 100 people). In some disease states, such as cancer, patients will be studied instead of healthy volunteers. Participants are under strict medical supervision. 

The purpose of Phase 1 clinical trials is to test: 

  • the investigational drug’s safety, 
  • how it affects the body, 
  • how it is tolerated, and 
  • if it causes any unwanted side effects. 

Phase 2

Larger groups of patients (up to several hundred people) participate in Phase 2 clinical trials. At this stage, researchers study how the investigational drug or vaccine is tolerated in patients. 

The purpose of Phase 2 trials is to study:

  • any short-term side effects,
  • what the optimal dose of the investigational drug or vaccine might be, 
  • how many times it should be given, over what period of time. 

In Phase 2 trials, one group of patients may be given the investigational drug or vaccine, while the other may be given a placebo or other approved treatment.

Phase 3

Larger groups of patients (typically several hundred to thousands of people) participate in Phase 3 clinical trials. These studies can take many years to complete. They are often carried out in many different locations. 

The purpose of a Phase 3 trial is to determine:

  • whether the investigational drug or vaccine is safe, and 
  • whether it has a therapeutic (beneficial) effect. 

In this type of study, researchers compare the investigational treatment with the current standard treatment or placebo. They monitor any side effects very closely.

In most cases, a participant will not know whether they have received the investigational drug or vaccine, an existing treatment, or placebo. The medical professionals will often not know, either. This is called a "double blind" trial. It helps minimize any bias or prejudiced opinion, and helps researchers draw conclusions with confidence.


Once the three phases have been completed successfully, the sponsoring company submits the findings to health authorities for approval. In the United States, this authority is the Food and Drug Administration (FDA). 

The FDA reviews all the findings, from both the clinical trials and earlier laboratory studies, as well as other important product information. Based on its review, the FDA will decide whether the drug or vaccine should be approved. 

If the drug or vaccine is approved, it can be made available for doctors to prescribe to patients as appropriate. 

Phase 4

Research does not necessarily stop after a medicine or vaccine is approved. Studies continue so that researchers can learn about how it affects people over time. Phase 4 trials generally include very large numbers of patients. They often resemble usual medical care settings. 

The purpose of a Phase 4 clinical trial is to:

  • detect any possible rare, undesirable side effects that may occur over time; and 
  • evaluate how the drug is used in combination with other treatments.


MAT-US-2105222-v1.0-10/2021 | Last update October 2021
Photo: Getty Images