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All About Clinical Trials

The Phases of a Clinical Trial

To find out whether a treatment is efficacious and safe, clinical trials are typically done in three phases. They may sometimes include a fourth phase.

First, the potential study treatment (drug, medical device, diagnostic tool, surgical procedure, vaccine...) is tested on a few people to look at its safety (Phase I). Then, if the results are positive, the treatment is tested on a small number (Phase II) and then on a large number (Phase III) of people with a specific medical condition to test the treatment’s efficacy and safety. Even after the treatment is available on the market, it is monitored while in use by the public. Phase IV trials can sometimes be conducted during this period.

The Distinct Objectives for Each Phase

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The Distinct Objectives for Each Phase

Phase I

In Phase I trials, the treatment in question is usually tested in healthy people. In some cases, however, the treatment may be tested directly on people with a specific medical condition.

Several doses of the treatment are usually tested (using different groups of participants) to find out the maximum dose. Scientists can also test the best way to administer the treatment (orally, by injection...). They study what happens to the drug in the body and watch for possible side effects.

  • The main goal is to find out whether the treatment generally is safe for humans.
  • In Phase I, the treatment is given to a group of about 15 to 30 people.
  • Phase I trials last 1 to 2 years.
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Phase II

In Phase II trials, the study treatment is tested on people with a specific medical condition.

  • The objectives are to analyze the preliminary efficacy of the treatment for the medical condition being studied, continue assessing the safety of the treatment, monitor the potential occurrence of side effects, and determine the optimal dosage to administer.
  • In Phase II, the study treatment is given to a group of about 100 people.
  • Phase II trials last 2 to 3 years.
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Phase III

In Phase III trials, the treatment is compared with a standard treatment (or standard of care) commonly used for the medical condition being studied, or with a placebo.

  • The goals are to confirm the efficacy of the treatment, find out whether it is more beneficial than the standard of care or a placebo and continue to assess its safety by testing it on more people.
  • In Phase III, the treatment is given to a group of between 100 and several thousand people.
  • Phase III trials last 1 to 4 years.

Note: Phase I and II or II and III trials are sometimes conducted at the same time or are combined.

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Phase IV

After the treatment is approved, Phase IV studies are sometimes conducted when requested by regulators, manufacturers or doctors, when the treatment is available to the public.

Phase IV studies confirm that the drugs are safe and effective under real-world conditions of use by the public.

  • The objectives include evaluating the safety, effectiveness, and efficiency of the new treatment in real-world conditions of use by a large number of people, and obtaining information about its long-term benefits and disadvantages.
  • In Phase IV, information is usually collected from hundreds or even thousands of people.
  • Phase IV studies last 1 to 4 years.

Note: For clinical trials involving rare diseases, the duration can vary widely.

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