Phentermine works as an appetite suppressant and can help people lose weight in combination with diet and exercise.

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.

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).

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.

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

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.

WHAT IS IT USED FOR?

Phentermine is most often used for weight loss, but may also be used in combination with the medication topiramate to help with certain eating disorders, such as binge eating disorder (BED) (8Trusted Source).

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

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

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

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

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

SUMMARY

Phentermine combined with topiramate has been shown to promote significant weight loss and may help reduce the incidence of binge-eating episodes.

Dosage and forms

Dosages for phentermine vary depending on its form and concentration.

Phentermine

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

However, since it’s a best practice for doctors to prescribe the lowest effective dose, the FDA approved an 8-mg formulation (Lomaira) in 2016, which can be taken up to three times daily at least 30 minutes before meals.

If you’re prescribed a higher dose (15, 30, or 37.5 mg), you can take it once per day in the morning before breakfast or 1–2 hours after breakfast.

To prevent insomnia or difficulty falling or staying asleep, you should avoid taking the last dose too late in the day.

Phentermine and topiramate

The combination of phentermine and topiramate — sold under the brand name Qsymia — is a medication used for weight loss.

This medication is available in four doses, ranging in strength from 3.75–15 mg of phentermine and 23–92 mg of topiramate. It’s offered in an extended-release capsule that should be taken in the morning

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

According to the FDA, you should stop using the medication if you don’t lose 3% of your body weight after 12 weeks on the highest daily dose

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 (

The most commonly reported side effects of phentermine and combined phentermine and topiramate are

• 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

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

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