Phentermine is the most widely-prescribed weight-loss medication in the United States. It first received FDA approval in 1959 as an appetite suppressant for the short-term management of obesity. Phentermine helps patients lose weight by decreasing appetite and boosting energy. How Does Adipex Work For Weight Loss? In a basic explanation, phentermine pills are used like a stimulant to the central nervous system – it helps to suppress appetite and increase energy, making it somewhat easier to reduce caloric intake and boost the metabolism to burn fat cells. Phentermine is a popular and effective weight loss medication, but unfortunately many users report that phentermine stopped working after 2-3 months of use. The main ingredient in aspirin, salicin, is the oldest pain reliever in history, but it took time and science before it became the drug we recognize today. Phentermine is a prescription weight loss medication. It was approved by the FDA in 1959 for short-term use of up to 12 weeks for people older than 16 (). Phentermine (contracted from phenyl-tertiary-butyl amine), also known as α,α-dimethylphenethylamine, is a psychostimulant drug of the substituted amphetamine chemical class, with pharmacology similar to amphetamine. Doctors prescribe phentermine only for people who are truly obese, with a body mass index of 30 or more and medical--not cosmetic--indications for weight loss. Like other prescription weight-loss drugs, phentermine is intended to be used as part of an overall weight-loss plan. It is indicated for people who are obese, and who have failed to lose enough weight with diet and exercise alone — not for people who want to lose just a few pounds. Phentermine for weight loss is a prescription-only drug generally prescribed to the severely overweight or obese individuals who also have a high risk for contributing health factors. Phentermine is an appetite suppressant used together with diet and exercise to treat obesity. Learn about side effects, interactions and indications.