Generic name: phentermine [ FEN-ter-meen ]
Brand names: Adipex-P, Lomaira
Drug classes: Anorexiants, CNS stimulants

Medically reviewed by Philip Thornton, DipPharm. Last updated on Nov 9, 2023.

What is phentermine?

Phentermine is used to speed weight loss in overweight people. It is used together with diet and exercise to treat obesity, especially in people with risk factors such as high blood pressure, high cholesterol, or diabetes.

Phentermine is similar to an amphetamine. It stimulates the central nervous system (nerves and brain), which increases your heart rate and blood pressure and decreases your appetite.

Phentermine belongs to a class of medications called anorectics. It works by reducing your appetite.

Warnings

Do not use phentermine if you are pregnant or breastfeeding a baby.

You should not use phentermine if you have glaucoma, overactive thyroid, severe heart problems, uncontrolled high blood pressure, advanced coronary artery disease, extreme agitation, or a history of drug abuse.

Do not use this medicine if you have used an MAO inhibitor in the past 14 days, such as isocarboxazid, linezolid, methylene blue injection, phenelzine, rasagiline, selegiline, or tranylcypromine. A dangerous drug interaction could occur.

Before taking this medicine

You should not use phentermine if you are allergic to it, or if you have:

• a history of heart disease (coronary artery disease, heart rhythm problems, congestive heart failure, stroke);

• severe or uncontrolled high blood pressure;

• overactive thyroid;

• glaucoma;

• extreme agitation or nervousness;

• a history of drug abuse; or

• if you take other diet pills.

Do not use phentermine if you have used an MAO inhibitor in the past 14 days. A dangerous drug interaction could occur. MAO inhibitors include isocarboxazid, linezolid, methylene blue injection, phenelzine, rasagiline, selegiline, tranylcypromine, and others.

Weight loss during pregnancy can harm an unborn baby, even if you are overweight. Do not use phentermine if you are pregnant. Tell your doctor right away if you become pregnant during treatment.

You should not breast-feed while using this medicine.

To make sure this medicine is safe for you, tell your doctor if you have:

• heart disease or coronary artery disease;

• a heart valve disorder;

• high blood pressure;

• diabetes (your diabetes medication dose may need to be adjusted); or

• kidney disease.

Phentermine is not approved for use by anyone younger than 16 years old.

How should I take phentermine?

Take phentermine exactly as prescribed by your doctor. Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose.

Doses are normally taken before breakfast, or 1 to 2 hours after breakfast. Follow your doctor's dosing instructions very carefully.

Never use phentermine in larger amounts, or for longer than prescribed. Taking more of this medication will not make it more effective and can cause serious, life-threatening side effects.

This medicine is for short-term use only. The effects of appetite suppression may wear off after a few weeks.

Phentermine may be habit-forming. Misuse can cause addiction, overdose, or death. Selling or giving away this medicine is against the law.

Call your doctor at once if you think this medicine is not working as well, or if you have not lost at least 4 pounds within 4 weeks.

Do not stop using this medicine suddenly, or you could have unpleasant withdrawal symptoms. Ask your doctor how to safely stop using this medicine.

Store at room temperature away from moisture and heat. Keep the bottle tightly closed when not in use.

Dosing information

Usual Adult Dose for Obesity:

8 mg orally 3 times a day 30 minutes before meals, OR
15 to 37.5 mg orally once a day before breakfast or 1 to 2 hours after breakfast.

Use: Short-term (a few weeks) adjunct in a regimen of weight reduction based on exercise, behavioral modification, and caloric restriction in the management of exogenous obesity in patients with an initial body mass index (BMI) of 30 kg/m2 or greater, or BMI of 27 kg/m2 or greater in the presence of other risk factors (e.g., controlled hypertension, diabetes, hyperlipidemia).

Usual Pediatric Dose for Obesity:

Age 17 Years and Older:
8 mg orally 3 times a day 30 minutes before meals, OR
15 to 37.5 mg orally once a day before breakfast or 1 to 2 hours after breakfast.

Use: Short-term (a few weeks) adjunct in a regimen of weight reduction based on exercise, behavioral modification, and caloric restriction in the management of exogenous obesity in patients with an initial body mass index (BMI) of 30 kg/m2 or greater, or BMI of 27 kg/m2 or greater in the presence of other risk factors (e.g., controlled hypertension, diabetes, hyperlipidemia).

What happens if I miss a dose?

Take the medicine as soon as you can, but skip the missed dose if it is late in the day. Do not take two doses at one time.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. An overdose of phentermine can be fatal.

Overdose symptoms may include confusion, panic, hallucinations, extreme restlessness, nausea, vomiting, diarrhea, stomach cramps, feeling tired or depressed, irregular heartbeats, weak pulse, seizure, or slow breathing (breathing may stop).

What to avoid

Avoid driving or hazardous activity until you know how phentermine will affect you. Your reactions could be impaired.

Drinking alcohol with this medicine can cause side effects.

Phentermine side effects

Get emergency medical help if you have signs of an allergic reaction to phentermine: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Phentermine may cause serious side effects. Call your doctor at once if you have:

• feeling short of breath, even with mild exertion;

• chest pain, feeling like you might pass out;

• swelling in your ankles or feet;

• pounding heartbeats or fluttering in your chest;

• tremors, feeling restless, trouble sleeping;

• unusual changes in mood or behavior; or

• increased blood pressure - severe headache, blurred vision, pounding in your neck or ears, anxiety, nosebleed.

Common side effects of phentermine may include:

• itching;
• dizzinessheadache;
• dry mouth;
• unpleasant taste;
• diarrhea;
• constipation;
• stomach pain; or
• increased or decreased interest in sex.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Related/similar drugs

What other drugs will affect phentermine?

Taking phentermine together with other diet medications such as fenfluramine (Phen-Fen) or dexfenfluramine (Redux) can cause a rare fatal lung disorder called pulmonary hypertension. Do not take phentermine with any other diet medications without your doctor's advice.

Many drugs can interact with phentermine. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed here. Tell your doctor about all your current medicines and any medicine you start or stop using.

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Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use phentermine only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

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