What Does the “Phase” of a Clinical Trial Mean?

What Does the “Phase” of a Clinical Trial Mean?


New treatments must pass through different phases before they can be released to the general public. If the treatment is successful in one phase, it will move to the next one. Patients typically participate in only one phase of a trial.

Phase I Clinical Trials

The first step in testing a new treatment is called a phase I trial. Phase I trials look at a new treatment to determine how well it works against a particular type of cancer, along with safety, best uses, dosage, and frequency of dosage.

These early tests can carry significant risks. While the treatments that are studied are sometimes effective, there is no guarantee that they will work. For these reasons, phase I studies usually involve a small group of 20 to 80 patients.

The first few people in a phase I study usually get a low dose of the new treatment. If there are only minor side effects, or none at all, the next few patients will get a slightly higher dose. This process continues until doctors find the dose that is most likely to work without causing severe side effects.

Like all trials at MSK, patients enrolled in phase I studies benefit from close collaboration with doctors and scientists and receive a high level of oversight and care to ensure patient safety. The Early Drug Development Service includes phase I trials and specializes in bringing effective drugs to patients faster.

Learn more about whether a phase I trial is right for you.

Phase II Clinical Trials

A phase II trial typically studies how well a new treatment works in a group of up to 100 patients with a similar type of cancer. Patients are treated using the dose and method found to be safest and most effective in phase I studies.

Some phase II studies randomly assign patients to one of two treatment groups. This is called randomization and it helps doctors learn which approach provides the best balance of safety and effectiveness. These groups may receive different treatments, different doses of the same drug, or the same treatment delivered in different ways.

In many phase II randomized cancer trials, one group will receive the standard treatment while the other group receives the standard treatment plus the new treatment. Phase II trials may also look at the quality of life benefit offered by a particular treatment.

In some cases, if a drug is successful in a phase II trial, the researchers may apply to the US Food and Drug Administration (FDA) to market it for doctors to treat the general public.

Phase III Clinical Trials

Phase III clinical trials compare new treatments with standard treatments, or compare new treatments with medical records from a group of patients who were observed in the past. In phase III trials, researchers are commonly trying to see which approach is safer and more effective.

A phase III trial can include hundreds to thousands of patients. Each participant is randomly assigned to receive either the new treatment or standard therapy.

In phase III studies, doctors typically look to see whether the new treatment gives patients a longer life, a better quality of life, and fewer side effects, and also shows fewer cases of cancer returning than standard treatment.

In some cases, a phase III clinical trial can be stopped if the researchers notice that one group is doing significantly better than the other. If a drug is successful in a phase III trial, the researchers may apply to the US Food and Drug Administration (FDA) to market it for doctors to treat the general public.

Phase IV Clinical Trials

After the FDA approves a treatment, it may be studied further in a phase IV trial. Phase IV trials evaluate the side effects, risks, and benefits of a drug or other therapy over a long period of time and in a larger number of people than in phase III trials. Thousands of people are involved in a phase IV trial.

MSK does not perform phase IV clinical trials.