Phentermine acts on your brain and nerves, which make up your central nervous system. It belongs to a class of medicines called sympathomimetic amines, which are similar to amphetamines. Phentermine works by stimulating your brain and nerves, and it has these effects:

• It increases your heart rate.
• It increases your blood pressure.
• It decreases your appetite.

Unfortunately, the effects of Phentermine do wear off after a few weeks, but it can be a great help in getting your weight loss going during those early, tough weeks of a weight loss program. Doctors usually prescribe it for 3 to 6 weeks.