Phentermine is a weight loss drug that helps suppress appetite. No prescription can replace a healthful diet and exercise. However, phentermine may help those unable to lose weight through diet and exercise alone.

Phentermine can boost weight loss in some people, though results can vary from person to person. It depends on their reaction, how long they use it, and their weight loss activities.

Some people should not take phentermine for weight loss. For them, the risks outweigh the potential benefits.

In this article, we look at what the current research says about phentermine, if it helps with weight loss, recommended dosages, and how to use it.

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A person should discuss options for weight loss with their doctor.

Phentermine is a short-term weight loss drug that the Food and Drug Administration (FDA) have approved. The National Library of Medicine note that people should only use the drug for a few weeks.

Phentermine stimulates certain brain chemicals that tell the body it is full. This may curb appetite sooner and help a person eat less.

It is a complementary therapy for people who find it hard to lose weight after they make the diet and lifestyle changes to support a moderate weight.

In many cases, phentermine helps people lose weight. Since its approval in 1959, it remains one of the most popular prescription weight loss drugs, either alone or as part of a combination therapy.

The exact mechanism of how phentermine helps suppress appetite is unclear. Researchers suggest the drug increases neurotransmitter levels in the brain, which decreases hunger.

Weight loss results may also depend on the person’s overall changes to their diet and lifestyle. This includes regular exercise, calorie restriction, and a healthful diet.

These changes alone are often enough to cause weight loss, though phentermine may enhance these effects. For example, a study in Obesity notes that phentermine may boost the efforts of some diet programs.

This research notes that people who replaced one meal per day to reduce their calorie intake lost weight. And those who added phentermine to their program lost more weight and had fewer cravings.

Phentermine is for people with obesity or those unable to lose weight with a balanced diet and exercise alone. They should have a BMI over 30, or over 27 with a weight-related condition.

Importantly, a person should still make dietary and lifestyle changes to support their weight loss for the drug to work correctly.

Phentermine is not for people who wish to lose a few pounds or those who are otherwise healthy.

Doctors will tailor each dose so that a person takes the lowest effective amount.

Phentermine is also a short-term drug. Some studies report the safety of phentermine in long-term use, but it is still intended for a few weeks of use.

There are risks with phentermine, and a doctor will discuss them before they prescribe the drug for weight loss.

Side effects

Phentermine alone or in combination with other drugs, such as topiramate, may have their own risk of side effects, including:

  • trouble sleeping or staying asleep
  • chest pain
  • dizziness
  • increased blood pressure
  • constipation
  • diarrhea
  • vomiting
  • restlessness
  • heart palpitations
  • unpleasant taste
  • dry mouth
  • swelling in the legs and ankles
  • shortness of breath

In some cases, these drugs may also make someone unable to drive or operate heavy machinery. Doctors recommend that people take time away from work when they first start the drug, to gauge any side effects.

Other conditions or medications

A person’s conditions or medications may mean phentermine is unsuitable for them as a weight loss drug. These include:

Pregnant or breastfeeding women should also avoid phentermine until they stop breastfeeding.

Doctors will monitor people with diabetes or hypoglycemia who take phentermine. They may need to reduce their insulin or other medications when they take the drug.

Phentermine could also interact with alcohol, so a person on the drug should try to avoid alcoholic drinks.

Risk of abuse and tolerance

Phentermine is chemically related to amphetamine drugs, such as methamphetamine, a stimulant with a history of abuse. As such, there is potential for abuse and dependence when someone takes phentermine.

Some people may become tolerant to the drug, and want more to feel the same effects. In these cases, it is important to discontinue phentermine altogether, and move on to other methods to lose weight.

People who take prolonged doses may also experience withdrawal symptoms when they stop the drug. Talk to a doctor about the risk of withdrawal in each case.


A person who takes too much phentermine may overdose. Symptoms include:

  • hyperactivity
  • rapid breathing
  • psychosis
  • confusion
  • hallucinations
  • panic attacks
  • fatigue
  • depression
  • nausea
  • rapid heartbeat
  • high or low blood pressure
  • convulsions
  • coma

Phentermine, or a medicine that contains a mix of phentermine and other drugs, typically comes in an oral capsule. Doctors may tailor the amount to ensure a person takes the minimum effective dose.

Typical doses of phentermine for adults contain 15 milligrams (mg), 30 mg, or 37.5 mg, which delivers 12 mg, 24 mg, 30 mg of phentermine, respectively.

A daily dose of phentermine and topiramate can vary from 3.75 mg phentermine and 23 mg topiramate, to 15 mg phentermine and 92 mg topiramate.

Some underlying conditions will also change dosage recommendations. People with impaired kidneys may require smaller doses to avoid additional stress on these organs. Others with late-stage kidney disease may not be able to take the drug.

Doctors do not recommend phentermine in older adults, as it may stress the kidneys. Children under 16 years should also avoid the drug, due to uncertain risks.

It is also not advisable to take the drug in the evening or at night, as this may cause insomnia.

Different forms and dosages of phentermine will vary in price. Health insurance may also help cover costs in some cases.

Phentermine can help people lose weight, but it is still a complementary treatment. A healthful diet and lifestyle changes should be the foundation for a moderate weight. General weight loss tips include:

  • Maintaining a healthful diet, and eating nutrient-dense foods, such as fruit, vegetables, whole grains, and lean proteins.
  • Engaging in regular physical exercise, including cardio and strength training.
  • Avoiding greasy or less nutrient-dense foods, such as fast food or packaged goods.
  • Avoiding added calories from sugar-sweetened beverages.

Learn tips about how to lose weight safely and naturally here.

Phentermine is an FDA-approved weight loss drug that remains popular due to its effectiveness. A person who takes it alone or with other drugs may help boost their weight loss.

Phentermine is for people with obesity who struggle to lose weight with diet and exercise alone. There are still risks with phentermine and similar drugs. A person who wishes to take them should see their doctor to discuss their options.