Phentermine raises the level of a chemical in the brain, called serotonin. Increases in serotonin have been shown to decrease depression and reduce calorie consumption.

A rare, potentially life-threatening condition called pulmonary hypertension has occurred in people who take a drug (Redux and Pondimin), similar to phentermine. An important and serious drug interaction can occur when combining phentermine with Prozac (or similar antidepressant drugs).