Phentermine is a drug that was part of the Fen-phen anti-obesity drug, the other drug being fenfluramine. Fenfluramine was withdrawn from the U.S. market in 1997 after reports of heart valve disease and pulmonary hypertension.

Phentermine is still available by itself in most countries, including the U.S.

Phentermine, like many other prescription drugs, work with neurotransmitters in the brain. It stimulates neuron bundles to release a particular group of neurotransmitters known as catecholamines; these include dopamine, epinephrine (formally known as adrenalin), and norepinephrine (noradrenalin). They signal a fight-or-flight response in your body which, in turn, puts a halt to the hunger signal. As a result, you lose your appetite because your brain doesn’t receive the hunger message. This is perhaps due to phentermine’s affects on leptin levels in the brain. It is theorized that phentermine can raise levels of leptin which signal satiety. It is also theorized that increased levels of the catecholamines are partially responsible for halting another chemical messenger know as neuropeptide Y. This chemical initiates eating, decreases energy expenditure, and increases fat storage.