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.

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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 ().

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 ().

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 (, , ).

Summary Phentermine is an FDA-approved drug intended for weight loss. Its chemical structure is similar to amphetamine, and it’s only available with a prescription.

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 (, ).

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.

Summary Phentermine is thought to decrease your appetite by increasing neurotransmitter levels in your brain.

Several clinical studies have proven that phentermine can boost fat loss.

The expected average weight loss with phentermine use is 5% of your initial body weight. Yet, over 12 weeks, it can be as high as 10%. This equates to a weight loss of 10–20 pounds (4.5–9 kg) for a 200 pound (90.7 kg) person ().

In a meta-analysis of six studies, people who took the average dose of 27.5 mg of phentermine for 13 weeks lost an average of 13.9 pounds (6.3 kg) compared to 6.2 pounds (2.8 kg) in placebo groups ().

While phentermine has been shown to be effective for weight loss, it may work better when combined with topiramate ().

Topiramate is a drug that has been used on its own to treat seizures but — like phentermine — also has appetite-reducing properties (, , ).

Topiramate and phentermine is a combination medication sold under the brand name Qsymia.

Compared to three other commonly prescribed drugs for weight loss, the combination of phentermine and topiramate was associated with the highest odds of losing at least 5% of the initial body weight ().

What’s more, research suggests that the phentermine and topiramate combination is the most effective weight loss medication to date — with people achieving an average weight loss of 21.6 pounds (9.8 kg) after taking the maximum dose for one year ().

In users, this weight loss has translated to a significant decrease in waist circumference, improved insulin sensitivity, and blood sugar control, as well as favorable effects on triglyceride and cholesterol levels (, ).

Summary Studies confirm the effectiveness of phentermine for weight loss. What’s more, the effect is even greater when the drug is combined with topiramate.

The combination of phentermine and topiramate may help reduce binge eating in people with binge eating disorder (BED) and bulimia nervosa.

BED is characterized by eating large amounts of food, often quickly and to the point of discomfort. It’s also associated with a feeling of losing control during the binge, as well as feelings of shame or guilt afterward ().

Bulimia nervosa involves the same binge-eating behavior as with BED, but includes behaviors, such as self-induced vomiting, in an attempt to compensate for the effects of binge eating ().

In a small 12-week study in obese or overweight people with BED, the phentermine and topiramate drug combination was associated with significant reductions in weight, BMI, and binge-eating episode frequency ().

In another 12-week study, people with BED or bulimia nervosa were randomized to receive the drug combination or a placebo.

Over 28 days, treatment with the phentermine and topiramate combination decreased the participants’ number of binge-eating days from 16.2 to 4.2. The same results were not observed in the placebo group ().

By reducing binging episodes, the drug combination may help the estimated 40–80% of people with BED or bulimia nervosa who are overweight or obese lose weight while improving mood and a sense of control with eating ().

Summary The phentermine and topiramate combination has been shown to reduce binging episodes and weight in people with BED and bulimia nervosa.

Dosages for phentermine vary depending on its form and concentration.

Phentermine

Prior to 2016, the available doses of phentermine were 15, 30, and 37.5 mg.

However, since it’s recommended that doctors prescribe the lowest effective dose, the FDA approved an 8-mg formulation in 2016, which can be taken up to three times daily.

You should avoid taking the last dose too late in the day to prevent insomnia or difficulty falling or staying asleep.

Phentermine and Topiramate

Phentermine and topiramate — sold under the brand name Qsymia — is a combination medication used for weight loss (, ).

This medication is available in four doses, ranging in strength from 3.75 to 15 mg of phentermine and 23 to 92 mg of topiramate.

After taking the lowest dose for 14 days, your doctor can choose to progress you to a higher dose.

The medication should be discontinued if you don’t lose 5% of your body weight after 12 weeks on the highest daily dose.

Summary The dosage of phentermine differs, depending on whether it’s used alone or alongside topiramate.

Phentermine alone is designed for short-term use only, as there are no long-term studies on its safety.

However, the FDA has approved phentermine in combination with topiramate for long-term use, since the dosage of the two ingredients is lower than the maximum doses of the individual drugs ().

While serious adverse effects are rare, studies report several side effects of the phentermine and topiramate combination ().

The most commonly reported side effects include (, , ):

  • Dry mouth
  • Sleeping problems
  • Dizziness
  • Heart palpitations
  • Flushing of the skin
  • Fatigue
  • Constipation
  • Irritability

You shouldn’t take phentermine if you have heart disease, hyperthyroidism, glaucoma, or if you’re pregnant or nursing ().

Phentermine should also not be prescribed in combination with monoamine oxidase inhibitors (MAOIs), a class of medications used to treat depression.

Your doctor will determine whether phentermine is appropriate and safe for you.

Summary While there are common side effects associated with the use of phentermine, it’s tolerated by most people. However, people with certain conditions and women who are pregnant or nursing should not use phentermine.

While phentermine can be a powerful weight loss aid, the only proven way to shed weight — and to keep it off in the long term — is cultivating healthy lifestyle behaviors ().

Without making the proper changes, it is likely that you’ll gain back the weight you lost — and possibly more — once you stop taking phentermine.

A comprehensive lifestyle change includes:

  • A reduced-calorie diet: If you have excess weight to lose, eat 300–500 fewer calories per day. A registered dietitian can help you tailor this range based on your preferences and goals ().
  • Prioritize nutrient-dense foods: Nutrient-dense foods — such as fruits and vegetables — are relatively low in calories and high in nutrients, such as fiber, vitamins, and minerals.
  • Increase physical activity: Guidelines recommend a minimum of 150 minutes per week of moderate aerobic exercise, such as brisk walking or running (, 25).
  • Behavioral strategies: Behavior changes includes regular self-monitoring of food intake, physical activity, and your weight, plus making adjustments as needed ().

Making these lifestyle changes can be difficult and shouldn’t happen all at once. It will take an investment of your time and energy — but the outcome will be long-term weight loss and overall better health.

Summary Lifestyle and behavioral modification are the cornerstones of successful weight loss and maintenance.

Phentermine is a prescription-only appetite suppressant and weight loss pill, approved for short-term use.

The combination of phentermine and topiramate appears to be even more effective and tolerable than phentermine alone.

Side effects include dry mouth, dizziness, fatigue, irritability, and constipation.

The weight loss benefits of phentermine and topiramate also expand to people with BED and bulimia nervosa.

While phentermine can be a useful short-term weight loss tool, you must make healthy lifestyle changes for long-term success.