What are Clinical Trial Phases?

Clinical trials are carried out to test the effectiveness and safety of an investigational drug or medical device on people. Researchers move on to the human clinical trial stage after they have already performed pre-clinical research including animal studies on the investigational drug, and it's determined that the drug is potentially beneficial enough to be tried with human participants. There are five clinical trial phases to ensure the drug is both safe and effective.

The U.S. Food and Drug Administration (FDA) explains that it looks at pre-clinical testing results and human testing proposals and determines whether clinical trials with people are reasonably safe. Then, the FDA and an institutional review board (IRB) review the investigational new drug application, and the IRB may approve the clinical trial protocols that determine how the trials will be carried out.

Once the investigational treatment has moved to clinical trials, the process goes through five phases from Phase 0 to Phase 4. These clinical trial phases include the following:

Phase 0

Initially, researchers carry out a limited phase on a small group of about 10 to 15 participants. These Phase 0 trials look at the investigational drug's effects on the body and how the body processes it. This is an exploratory period that does not focus on effectiveness.

Nonetheless, it may look at how the body responds to the drug in ways that relate to the specific health condition or disease the drug is designed to treat. For example, the American Cancer Society explains that a cancer drug Phase 0 study may look to see if the investigational drug reaches a cancerous tumor or how actual cancer cells react to the drug in the body.

Phase 1

In Phase 1 clinical trials, researchers test the drug on about 20 to 100 participants who are usually healthy. In this phase, they study different drug doses and their effects for several months. They give small doses at first and then increase the doses until they achieve the desired result or they need to stop due to side effects.

The goal is to find the safest dose that works with minimal side effects, to determine frequent and serious side effects, and to see how the body metabolizes and excretes the drug. Researchers find the safest dose and make sure there is not unacceptable toxicity from the drug before moving to the next phase.

Phase 2

Phase 2 clinical trials continue to look at safety while also collecting preliminary information on the effectiveness of the investigational drug in relation to the specific health condition or disease. The drug is given to a range of about a few dozen to a few hundred participants who have the health condition or disease to see if it works and at what dose. This phase is carried out over several months up to two years.

If it's a “controlled” clinical trial, the study will compare these results to a similar group of participants who receive an inactive placebo or another treatment. Researchers continue to look for safety and short-term side effects. Some evidence of effectiveness needs to be shown in this phase before the clinical trial would move on to Phase 3.

Phase 3

Phase 3 clinical trials take research on safety and effectiveness of the investigational drug to another level, ensuring the drug is effective and monitoring side effects in a larger population for a longer period of time. They gather more data on using the drug in various doses and in different populations, studying 300 to 3,000 participants over the course of one to four years.

They may also study the use of the drug combined with the use of other drugs, and they may compare the efficacy and safety of the drug to the standard-of-care drug that is currently used for the purpose. These are often randomized trials that separate people into different study groups randomly.

Phase 4

In Phase 4 clinical trials, studies continue on the drug after the FDA has already approved it for marketing. Part of this phase is to carry out post-market requirement and commitment studies that the study sponsor committed to or must conduct, explains the National Institutes of Health U.S. National Library of Medicine. Sponsors are those taking responsibility for the drug's development and can include research institutions, companies or other organizations.

During Phase 4, researchers continue to study the effectiveness, safety, and/or optimal use of the drug. This phase includes thousands of participants who all have the health condition or disease for which the drug is intended.

These five clinical trial phases show how new drugs are carefully studied in increasingly larger numbers of people over an extended period of time. The phases help ensure a drug is safe and effective before moving on to the next phase and before being put on the market. Even after it's on the market, a new drug continues to be studied to ensure safety and effectiveness in the broader population over the long term, and potential new uses.

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