What Are Phase I, II, III, and IV Clinical Trials?

Each clinical trial is conducted in four phases. The Food and Drug Administration (FDA) must approve each phase before the study can continue.

Phase I: In this phase, a new drug or treatment is tested on a small group of healthy people to determine safe dosage, study how the drug works in the body, and see if it has any side effects. The overall safety of the drug is not known during this phase.

Phase II: The drug or treatment can now be tested on a larger group of people to see if it is effective and to further test its overall safety. Rating scales are developed and used to record data during this phase.

Phase III: Now the drug or treatment is ready to be tested on even larger numbers of people. The study will look even more closely at the drug's effectiveness, if it has any side effects, overall safety, and how it can improve a person's quality of life. Most drugs that reach this phase are considered for FDA approval.

Phase IV: Once given FDA approval, the trial can enter into the final phase, which involves monitoring the drug after it has been released to the public. In this phase, researchers look for additional information such as risks, benefits, and optimal or additional uses of the drug. In some cases this phase is used to test the drug on a sub-group of people (such as patients over a certain age).

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