A phase 4 clinical trial begins after a drug has been approved for use in the general population following phase 1, 2 and 3 trials to rigorously test its efficacy and safety.

Prior to the phase 4 trial

In phase 1 clinical trials, the safety, tolerability and toxicity of the drug at different doses is tested in a small number of healthy volunteers. In phase 2, the drug's efficacy and optimal dosing regimen are established.

After phase 2, the drug enters the most vital stage of its development, the phase 3 trial. This is the last phase of testing the drug needs to complete before the drug information and clinical trial results are submitted to the regulatory authorities for approval of the drug for use in the general population.

Phase 4 trial

The phase 4 trial is also referred to as post marketing surveillance and as the name suggests, it is conducted after the drug is already marketed and available to the general public. The main objective of the phase 4 trial is to check the drug's performance in real life scenarios, to study the long-term risks and benefits of using the drug and to discover any rare side effects.
 

In a phase 4 trial, any rare or long-term effects of the drug can be observed in a much larger population of patients and over a much longer period of time. If safety surveillance does indeed reveal concerns about the drug, it may be withdrawn from the market and no longer made available on prescription.

Other reasons for conducting a phase 4 trial include:

Previous clinical trials may have been limited somewhat in how thoroughly they could test the various different factors that could influence the drug's performance. For example, clinical trial patients may be instructed to follow a strict diet and drug regimen, while phase 4 trials are conducted on regular populations among whom a variety of different foods and other drugs may be ingested.

Untoward effects may have been missed in earlier trials and it is important for phase 4 results to be submitted to the regulatory authorities so that any rare side effects that may have been missed earlier can be noted.

Phase 4 trials are conducted in varied populations across multiple centres which eliminates bias or any unfair influence that could affect the results of the studies.

Many drugs have shown harmful effects only during phase 4 trials and have subsequently been banned from use. One example is the pain reliever rofecoxib which showed negative cardiac side effects during phase 4 trials and was therefore withdrawn from the market.