Does Phentermine Burn Fat?

Obesity causes a number of serious health concerns, including heart disease and diabetes. For some people, the dangers of obesity override the potential for dangerous side effects of prescription weight-loss drugs such as phentermine. Although it's not entirely clear how phentermine works, it primarily acts by suppressing appetite.

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According to RX List, the diet drug phentermine is chemically related to amphetamines, which aid in weight loss by suppressing appetite 1. Its true chemical name is alpha-alpha-dimethylphenethylamine hydrochloride. Adipex-P is one common trade name for phentermine 1.


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. Patients should use daily phentermine tablets as part of an overall weight-loss plan, including calorie restriction, behavioral modification and an exercise regimen. Patients take phentermine for no more than about 10 weeks, according to Rx List 1.


The National Institute of Diabetes and Digestive and Kidney Disorders classifies phentermine as an appetite suppressant, because doctors know that phentermine inhibits hunger by interfering with chemical messengers in the part of the brain that controls appetite. The Merck Manual and RX List explain that the effects of phentermine on the body are not completely described, and that it might also increase metabolism and make the body use more energy than normal 1. Increasing the metabolic rate would help the body “burn fat," but there is no solid clinical evidence showing that phentermine significantly affects the metabolic rate. Among all of the medications approved by the FDA for weight loss and listed by the National Institute of Diabetes and Digestive and Kidney Disorders, not one of them acts by increasing the metabolic rate.


According to Rx List, research shows that a combination of phentermine and calorie restriction promotes more weight loss than placebo and calorie restriction 1. However, the difference is only “a fraction of a pound” per week. Because phentermine is meant for short-term use, its long-term effects on sustained weight loss might not be significant. The National Institute of Diabetes and Digestive and Kidney Disorders says that most people who stop taking weight-loss medications ultimately regain the weight they lost during treatment.


Phentermine can cause dangerous side effects, including heart disease, addiction and rarely, psychotic episodes. If the drug stops working over time, patients should alert their doctors and never take more than the prescribed amount.