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Originally Posted by Monticore
Some even get pulled years later after being approved.
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Yes, and not many people know why.
There are actually four phases to clinical trials. You get approved after Phase III, but the fourth phase (the silent phase) is called post-marketing surveillance. Because of the nature of clinical trials, you cannot enroll enough patients to detect truly rare outcomes of certain drug therapies. Thus, after a drug has been on market, serious and novel adverse effects are reported to the FDA to catch more of these in its net as the sample size of patients increases. Eventually, you build enough data to have a better understanding of the drug's effects on all aspects of the population rather than the much more limited scope offered by even multicenter Phase III trials.
A great example: terfenadine was a non-sedating antihistamine that was metabolized in the body to fexofenadine (Allegra). The metabolite is not toxic, but terfenadine blocks a potassium channel which leads to arrhythmia. If used in combination with other CYP3A4 enzyme inhibitors, that effect is increased and can lead to life threatening arrhythmias. As a result, it was pulled of the market years later, but it would have been difficult to catch that in a Phase III.