Phase 4 clinical trials look at drugs that have already been approved by the FDA. Even though a drug may be prescribed to patients, this type of clinical trial is still important for answering questions. Phase 4 clinical trials usually involve thousands of people. It is usually the safest type of clinical trial because the medication has already been approved. These studies will usually look at other aspects of the treatment process, including how cost-effective it is. This type of study may also be conducted to evaluate whether the medication is having an impact on someone's overall quality of life.