Phentermine is approved as an appetite suppressant to help reduce weight in obese patients when used short-term and combined with exercise, diet, and behavioral modification. It is typically prescribed for individuals who are at increased medical risk because of their weight and works by helping to release certain chemicals in the brain that control appetite.
Phentermine, like many other prescription drugs, works with neurotransmitters in the brain. It is a centrally-acting stimulant and is a constitutional isomer (not to be confused with stereoisomer) of methamphetamine. It stimulates neuron bundles to release a particular group of neurotransmitters known as catecholamines; these include dopamine, epinephrine (also known as adrenaline), and norepinephrine (noradrenaline). The anorectic activity seen with these compounds is thus likely due to their effect on the central nervous system, which is consistent with current knowledge about the central nervous system and feeding behavior. This is the same mechanism of action as other stimulant appetite suppressants such as diethylpropion and phendimetrazine. The neurotransmitters signal a fight-or-flight response in the body which, in turn, puts a halt to the hunger signal. As a result, it causes a loss in appetite because the brain does not receive the hunger message