With a low-calorie diet and regular exercise, phentermine is a prescription drug that is used to treat obesity, especially in people who have risk factors like diabetes, hypertension, or cardiovascular disease. Due to its chemical similarities to amphetamine, it is only available with a prescription. This medication primarily suppresses appetite effectively. It functions by promoting neurotransmitter release in the brain and is a member of the class of drugs known as sympathomimetic amines. Your ability to feel full and eat slightly decreases as these brain chemical levels rise.