The decision is this: FDA-approved diet pills are safe and effective. This may sound obvious, but you may remember a drug called Fen-Phen. It is one of the many FDA approved. After the FDA became aware of reports of heart damage, health professionals were notified of the results.
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The FDA is currently undergoing three phases of clinical trials for each drug seeking approval. First, tests are performed on a small group of people to determine possible side effects of the drug. At this stage, the effectiveness of the drug is also taken into account.
The second stage has a larger population, usually several hundred, and additional tests are done for side effects. It is at this stage that the effectiveness of the treatment of a particular condition is decided. For diet pills that are FDA approved, this is where diet pills really work.
The third phase of the study will be conducted on a large number of patients. The results are then extrapolated to determine the effectiveness and safety of the drug. The FDA then considered safety relative to effectiveness. Just one more step and you have an FDA approved diet pill.
The final step for the FDA is to evaluate the drug manufacturing process. They determine whether the procedure is sufficient to ensure the purity and integrity of the drug.