Phase IV clinical trials are also known as the Post Marketing Surveillance Trial.
Drugs in phase IV clinical trials have applied for (and received approval of) a New Drug Application (NDA).
The Federal Drug Administration is the agency responsible for approving drugs in Phase IV clinical trials. During this clinical research phase the drug or new medical treatment is usually being marketing to the public.
It continues to be studied for its interaction with other medications and potential uses other than the one that it was originally created for. During phase IV the drug continues to be monitored for safety surveillance, which is also known as pharmacovigilance. The long-term effects of the drugs are also monitored.
The Federal Drug Administration does not always require Phase IV clinical trials; however, sometimes it requires them as basis for approval. If they have doubts about the effects of a new drug on a larger population chances are they will require this clinical research trial.
During this time additional restrictions may be placed on a drug. For example it may be taken off the market for a certain part of the population (i.e. pregnant women). Sometimes drugs are taken off the market completely during phase IV trials. The cost-effectiveness of a drug is also studied during phase IV. Can patients afford to buy the medication in doses that will be effective for their treatment?
There is also something called pharamcoeconomics that looks at the cost of the drug or treatment in comparison to the quality of life that it can provide for patients.
Phase IV trials are an important final step in testing the safety and efficacy of a new drug or medical treatment in the general population. In most cases the medications act as they were expected and millions of people receive better treatment.