Phentermine: Drug Uses, Dosage & Side Effects

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

What is phentermine?

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 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 may also be used for purposes not listed in this medication guide.

Warnings

Do not use phentermine if you are pregnant or breast-feeding 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.

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.

Popular FAQ

Why am I tired on Phentermine?

Fatigue or tiredness can come as a result of sleepless nights or following a period of over-stimulation, however, adverse effects of phentermine also include drowsiness, fatigue, and reduced energy levels. If you’re feeling fatigued or lacking in energy while taking phentermine and the problem persists, you should consult your doctor as it may be that you have an undiagnosed medical condition.

Is phentermine safe to use?

Phentermine is safe to use when taken as prescribed by your doctor. Make sure your doctor knows about all of the other medicines you take, as well as any medical problems you have. It should not be taken along with other weight loss drugs. Though rare, high blood pressure has been reported in some patients.

Phentermine should not be taken if:

• You have cardiovascular disease
• You are taking or recently stopped taking drugs known as monoamine oxidase inhibitors
• You have hyperthyroidism
• You have glaucoma
• You have a history of drug abuse or agitation
• You are pregnant or nursing
• You are sensitive to or had a reaction to other stimulant drugs

How long does it take to start losing weight on phentermine?

Phentermine should be prescribed as part of a weight loss plan that also includes exercise, modifying behavior and reducing calories. Phentermine is usually taken for 12 weeks or less. Weight loss effects depend on the dose of phentermine taken, as well as diet, exercise and other factors.

How long does phentermine stay in your system?

Phentermine stays in your system for approximately 4 days after you take it. It takes about 20 hours for half of the drug to leave your system. Many things affect how long it takes a drug to leave your system. These include your age, diet, how well your kidneys and liver are working and other factors.

How to take phentermine for the best results?

Take phentermine as directed by your doctor. It is usually taken first thing in the morning, before breakfast. It may also be taken 1 to 2 hours after eating breakfast. Phentermine is not usually taken at night because it can cause insomnia.

Is Phentermine approved for weight loss?

Phentermine is a drug approved for weight loss as part of a regimen of exercise, calorie restriction and behavior modification. Phentermine may be prescribed for short-term treatment in people who have a body mass index (BMI) of 30 kg/m2 or greater. It may also be prescribed to people who have a BMI of 27 kg/m2 or greater with other risk factors, such as high blood pressure, diabetes or high cholesterol.

Some of the adverse effects of phentermine may include:

• Heart palpitations
• Raised blood pressure
• A feeling of restlessness
• Headaches
• Dry mouth
• Diarrhea
• Constipation
• Hives
• A higher or lower libido

How does phentermine work?

Phentermine affects the brain’s neurotransmitters. It is thought to cause weight loss by suppressing appetite. The exact way phentermine works to cause weight loss is not known. Phentermine belongs to the class of drugs known as sympathomimetic amines. These are more commonly known as stimulant drugs.

More about phentermine

• Check interactions
• Compare alternatives
• Pricing & coupons
• Reviews (2,983)
• Drug images
• Side effects
• Dosage information
• Patient tips
• During pregnancy
• Support group
• Drug class: anorexiants
• Breastfeeding
• En español

Patient resources

Other brands

Adipex-P, Lomaira, Fastin, Ionamin, … +3 more

Professional resources

Related treatment guides

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.

Copyright 1996-2023 Cerner Multum, Inc. Version: 10.01.