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While eating a well-balanced, reduced-calorie diet and exercising regularly are the cornerstones of weight loss, certain drugs can serve as powerful adjuncts.
One such drug is phentermine — one of the most popular weight loss drugs in the world.
It has been proven effective for short-term weight loss when used alongside a reduced-calorie diet and exercise.
However, using phentermine for weight loss is not without risks and side effects.
This article explains everything you need to know about phentermine, including its benefits, dosage, and possible side effects.
What Is Phentermine?
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 (1).
In the 1990s, phentermine was combined with other weight loss drugs. This drug combination was commonly called fen-phen.
After reports of significant heart problems in users, the FDA pulled the other two drugs used in the treatment — fenfluramine and dexfenfluramine — from the market (2).
Phentermine goes by the brand names Adipex-P, Lomaira, and Suprenza, or you can find it in combination medications for weight loss, such as Qsymia.
It’s a controlled substance due to its chemical similarities to the stimulant amphetamine — making it available only with a prescription.
Your doctor may prescribe phentermine if you’re obese, meaning that your body mass index (BMI) is greater than or equal to 30.
It may also be prescribed if you’re overweight with a BMI greater than or equal to 27 and have at least one weight-related condition, such as high blood pressure, high cholesterol, or type 2 diabetes (3, 4, 5).
How Does It Work?
Phentermine belongs to a class of drugs called “anorectics,” also known as appetite suppressants.
Taking phentermine helps suppress your appetite, thereby limiting how many calories you eat. Over time, this can lead to weight loss.
While the exact mechanisms behind the appetite-reducing effects of phentermine remain unclear, the drug is thought to act by increasing neurotransmitter levels in your brain (6, 7).
Neurotransmitters are your body’s chemical messengers and include norepinephrine, serotonin, and dopamine.
When your levels of these three chemicals increase, your feeling of hunger decreases.
However, you may build a tolerance to the appetite-suppressing effects of phentermine within a few weeks. In that case, you should not increase your dose of the drug but stop using it altogether.