All new drugs need proof that they are effective, as well as safe, before they can be approved for marketing. It's important to realize, however, that no drug is absolutely safe. There is always some risk of an adverse reaction. But the FDA uses a cost benefit analysis to determine if the drug can be released to the public. The FDA determines that when the benefits outweigh the risks the FDA considers a drug safe enough to approve.
Clinical research is divided into three phases in the normal course of testing.
Before a drug can be approved for sale to the public there is a set of clinical tests that must be performed. There is the Pre-Clinical Research Stage. Here the drug is synthesized and purified. Animal tests are performed, and institutional review boards assess the studies and make recommendations on how to proceed. If the recommendations are positive, then an application to the FDA occurs and clinical tests begin.