Phentermine works as an appetite suppressant and can help people lose weight in combination with diet and exercise. But the medication may not be safe for certain people.

Eating a well-balanced, reduced-calorie diet and exercising regularly are the cornerstones of weight loss, but certain drugs can provide powerful support.

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.

What is phentermine?

Phentermine is a prescription weight loss medication.

The Food and Drug Administration (FDA) approved it in 1959 for short-term use of up to 12 weeks for people older than 16 (1Trusted Source).

In the 1990s, drug makers combined phentermine 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 that were used in the treatment — fenfluramine and dexfenfluramine — from the market (2Trusted Source).

Today, phentermine can be used on its own and goes by the brand names Adipex-P, Lomaira, and Suprenza. You can also find it in combination medications for weight loss, such as Qsymia, which combines phentermine and another drug called topiramate.

Phentermine is a stimulant and is available if you have a prescription. Because taking phentermine comes with a risk of dependence, it is considered a controlled substance. It’s chemically similar to the stimulant amphetamine, which is also a controlled substance (3Trusted Source).

A healthcare professional may prescribe phentermine if you have obesity, defined by a body mass index (BMI) of 30 or greater.

Healthcare professionals may also prescribe it if you have excess weight 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 (4Trusted Source, 5Trusted Source).

However, a healthcare professional will likely recommend other weight loss strategies first, such as exercise and a calorie-reduced diet

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 way phentermine reduces appetite remains unclear, the drug is thought to act by increasing neurotransmitter levels in your brain (6Trusted Source, 7Trusted Source).

Neurotransmitters — the chemicals norepinephrine, serotonin, and dopamineare — are your body’s chemical messengers (6Trusted Source, 7Trusted Source).

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. If that happens, speak with the healthcare professional who prescribed it.

Effective for weight loss

Several clinical studies have shown that phentermine can support meaningful weight loss.

Though the FDA approved it only for short-term use of less than 12 weeks, healthcare professionals often prescribe phentermine off-label for longer use.

Doctors may prescribe it intermittently, meaning you take a break from the medication for a set period of time before resuming it (5Trusted Source, 9Trusted Source).

The average weight loss when taking phentermine is 3% of your initial body weight after 3 months and 5–7% after 6 months. Some people report even more weight loss than this (5Trusted Source, 6Trusted Source, 9Trusted Source, 10Trusted Source).

To put this into perspective, a 5–7% weight loss is 10–14 pounds (4.5–6.4 kg) for a person who weighs 200 pounds (90.7 kg).

However, not everyone responds the same way to this medication. If you haven’t lost much weight after 3 months of using it, a healthcare professional may recommend discontinuing the medication.

Further, its effectiveness appears to gradually decrease after prolonged use of more than a year (5Trusted Source, 6Trusted Source, 9Trusted Source, 10Trusted Source).

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 (6Trusted Source, 11Trusted Source, 12Trusted Source).

Topiramate and phentermine is a combination medication sold under the brand name Qsymia. The FDA approved it in 2012 as an addition to healthy lifestyle behaviors such as increased exercise and a calorie-reduced diet.

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 initial body weight (13Trusted Source).

What’s more, a recent analysis of studies found that people who took a medication that combined phentermine and topiramate lost 16 pounds (7.3 kg) on average compared with a placebo.

For those who took the medication for more than 56 weeks, this number increased to 17.8 pounds (8 kg) (14Trusted Source).

Furthermore, the degree of weight loss increased with the dose. Average weight loss was (14Trusted Source):

7.8 pounds (3.55 kg) for a 3.75/23 mg (phentermine/topiramate) dose

16 pounds (7.27 kg) for a 7.5/46 mg dose

18 pounds (8.25 kg) for a 15/92 mg dose

In users of phentermine-topiramate, weight loss translated to a significant decrease in waist circumference as well as improved insulin sensitivity, improved blood sugar management, and reduction in blood pressure (15Trusted Source, 16Trusted Source).

May be helpful for certain eating disorders

The combination of phentermine and topiramate may help reduce binge eating in people with 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 (17Trusted Source).

Bulimia nervosa involves the same binge-eating behavior as with BED but also includes behaviors people use to compensate for the effects of binge eating, such as self-induced vomiting (17Trusted Source).

In a small 12-week study in people with BED who had excess weight, people who took a phentermine and topiramate drug combination experienced significant reductions in binge-eating episode frequency (18Trusted Source).

In another 12-week study, people with BED or bulimia nervosa received the drug combination or a placebo (19Trusted Source).

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 (19Trusted Source).

While these results are promising, larger studies are still needed.

If you think you have BED, bulimia nervosa, or disordered eating, it’s important to speak with a healthcare professional.

Side effects and precautions

Phentermine alone is designed for short-term use only, as there are few long-term studies on its safety.

One study observed that people experienced no significant side effects when they took phentermine for up to 24 months (10Trusted Source).

However, the FDA has approved phentermine in combination with topiramate for long-term use, since the doses of the two ingredients are lower than the maximum doses of the individual drugs (20Trusted Source).

The most commonly reported side effects of phentermine and combined phentermine and topiramate are (6Trusted Source, 20Trusted Source, 21Trusted Source):

• dry mouth
• sleep problems
• dizziness
• heart palpitations
• skin flushing
• fatigue
• constipation
• irritability
• nausea
• vomiting

While rare, some severe side effects are possible, such as increased blood pressure, suicidal ideation, metabolic acidosis, increased creatinine levels, fetal harm, vision problems, cognitive impairment, and hypokalemia (6Trusted Source, 20Trusted Source, 21Trusted Source).

You shouldn’t take phentermine if you have heart disease, hyperthyroidism, glaucoma, or severe kidney or liver disease or if you’re pregnant or nursing (6Trusted Source, 20Trusted Source, 21Trusted Source, 22Trusted Source).

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

Speak with a healthcare professional if you’re taking other medications, such as antihypertensive or antiepileptic medications (20Trusted Source).

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

The bottom line

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 than phentermine alone, and it’s approved for long-term use. It may also help reduce bingeing episodes in people with BED.

Common side effects include dry mouth, dizziness, fatigue, irritability, and constipation. You should not use this medication if you have heart disease, hyperthyroidism, glaucoma, or severe kidney or liver disease or if you’re pregnant or nursing.

Also, be sure to consult a healthcare professional before trying phentermine if you’re taking certain medications, such as monoamine oxidase inhibitors (MAOIs).

While phentermine can be a useful weight loss tool, it’s not recommended as a first-line treatment. Instead, it’s best to prioritize healthy lifestyle changes for long-term success.