A Phentermine prescription given by a health care professional will usually consist of pills that will last for about 6-12 weeks. That is the acceptable term of medication as the body somehow adjusts to the effects of the drug after long term usage. If you are given a Phentermine prescription, it should be accompanied with a healthy diet and workout routine as well as behavioral modifications. A physician will not give you a Phentermine prescription if you are only overweight by a few pounds and is relatively healthy. This drug is generally given to obese patients who do not wish to undergo surgery.