They say that Phentermine is literally the most popular prescription diet pill out there. Generally speaking, Phentermine only works for a few weeks at a time. And yet people take it for this allotted period of time, take some time off, and then they use it again. If you're not sure, make sure to read phentermine reviews. There are alternative to this diet pills that might actually be better. Make sure that you do an extensive research and consult your doctor before try anything out.