Highlights for phentermine

  1. Phentermine oral capsule is available as a brand-name drug and as a generic drug. Brand name: Adipex-P.
  2. Phentermine comes in three forms: an oral capsule, an oral tablet, and an orally disintegrating tablet.
  3. Phentermine oral capsule is used to treat obesity.

Important warnings

  • Primary pulmonary hypertension (PPH) warning: This drug can cause PPH. This is a rare lung disease that may be fatal (cause death). PPH is a type of high blood pressure that affects the arteries in your lungs and the right side of your heart. Symptoms can include shortness of breath, heart palpitations (fast, fluttering heartbeat), dizziness, bluish color to your lips and skin, tiredness, and edema (swelling of your legs and ankles).
  • Valvular heart disease warning: This drug may harm your heart valves. Your valves may not be able to close properly and may leak. This may interrupt the blood flow through your heart to your body. Symptoms can include fatigue and weakness, shortness of breath during activity or when you lie down, swollen ankles and feet, chest pain, and an irregular or fast heartbeat.
  • Misuse and dependence warning: This drug may be habit-forming. You shouldn’t take it long-term. Tell your doctor if you have a history of drug or alcohol misuse before starting this drug.

What is phentermine?

Phentermine is a prescription drug. It comes as an oral capsule, an oral tablet, and an orally disintegrating tablet.

Phentermine oral capsule is available as the brand-name drug Adipex-P. It’s also available as a generic drug. Generic drugs usually cost less than the brand-name version. In some cases, they may not be available in every strength or form as the brand-name drug.

Phentermine is a controlled substance. Your use of this drug will be closely monitored by your doctor.

Why it's used

Phentermine oral capsule is only used short-term (for a few weeks) to treat obesity. This drug aids in weight loss in very obese and overweight people with certain health risk factors. These risk factors include high blood pressure, high cholesterol, and diabetes. This drug is used along with exercise, a low-calorie diet, and other behavior changes to lose weight.

How it works

Phentermine belongs to a class of drugs called anorectics. A class of drugs is a group of medications that work in a similar way. These drugs are often used to treat similar conditions.

It isn’t known exactly how phentermine works to help you lose weight. It may work by increasing the release of chemicals in your brain that help reduce your appetite. This may help you eat less, which would aid in weight loss.

Phentermine side effects

Phentermine oral capsule doesn’t cause drowsiness, but it can cause trouble sleeping. It can also cause other side effects.

More common side effects

The more common side effects of phentermine can include:

  • bad taste in your mouth
  • constipation
  • diarrhea
  • dry mouth
  • headache
  • vomiting

If these effects are mild, they may go away within a few days or a couple of weeks. If they’re more severe or don’t go away, talk to your doctor or pharmacist.

Serious side effects

Call your doctor right away if you have serious side effects. Call 911 if your symptoms feel life-threatening or if you think you’re having a medical emergency. Serious side effects and their symptoms can include the following:

  • Primary pulmonary hypertension (PPH). This is a type of high blood pressure that affects the arteries in your lungs and the right side of your heart. Symptoms can include:
    • shortness of breath
    • heart palpitations (fast, fluttering heartbeat)
    • dizziness
    • cyanosis (bluish color to your lips and skin)
    • tiredness
    • edema (swelling of your legs and ankles)
    • angina (chest pain)
  • Valvular heart disease. This drug may harm your heart valves so they can’t close properly and may leak. Symptoms can include:
    • fatigue and weakness, especially when you increase your activity level
    • shortness of breath during activity or when you lie down
    • edema (swelling of your legs and ankles)
    • angina (chest pain), chest discomfort or tightness that often gets worse during exercise
    • lightheadedness or fainting
    • arrhythmia (irregular heartbeat)
    • heart murmur
    • heart palpitations (fast, fluttering heartbeat)
  • Hypertension (high blood pressure)
  • Restlessness
  • Tremors (uncontrollable rhythmic movement in one part of your body)
  • Insomnia (trouble sleeping)
  • Erectile dysfunction in men (trouble getting or keeping an erection)

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs affect each person differently, we cannot guarantee that this information includes all possible side effects. This information is not a substitute for medical advice. Always discuss possible side effects with a healthcare provider who knows your medical history.

Phentermine may interact with other medications

Phentermine oral capsule can interact with other medications, vitamins, or herbs you may be taking. An interaction is when a substance changes the way a drug works. This can be harmful or prevent the drug from working well.

To help avoid interactions, your doctor should manage all of your medications carefully. Be sure to tell your doctor about all medications, vitamins, or herbs you’re taking. To find out how this drug might interact with something else you’re taking, talk to your doctor or pharmacist.

Examples of drugs that can cause interactions with phentermine are listed below.

Drugs you shouldn’t take

Taking certain drugs with phentermine may cause serious side effects. Don’t take these drugs with phentermine. Examples of these drugs include:

  • Monoamine oxidase inhibitors (MAOIs), such as isocarboxazid, phenelzine, rasagiline, selegiline, and tranylcypromine. You shouldn’t take phentermine within 14 days of starting or stopping an MAOI. Taking these drugs together can cause a hypertensive crisis (a dangerous increase in blood pressure).
  • Other drugs for weight loss, such as amphetamine, benzphetamine, dextroamphetamine, diethylpropion, and phendimetrazine. Taking these drugs together may cause increased side effects. These include hypertensive crisis (a dangerous increase in blood pressure) or arrhythmia (a serious abnormal heart rhythm).
  • Serotonin reuptake inhibitors (SSRIs), such as fluoxetine, sertraline, fluvoxamine, paroxetine, citalopram, escitalopram and nefazodone. Taking these drugs together can cause a hypertensive crisis (a dangerous increase in blood pressure).

Interactions that cause more side effects

Taking phentermine with certain medications may cause more side effects. Examples of these drugs include:

  • Insulin and oral diabetes drugs, such as nateglinide, pioglitazone, repaglinide, rosiglitazone, metformin, glimepiride, glipizide, sitagliptin, saxagliptin, linagliptin, exenatide, and liraglutide. Taking these drugs together may cause a significant drop in your blood sugar levels. Your doctor may lower your dosage of your diabetes drugs. You may also need to monitor your blood sugar levels closely.
  • Reserpine. Phentermine may decrease the ability of reserpine to lower your blood pressure.

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs interact differently in each person, we cannot guarantee that this information includes all possible interactions. This information is not a substitute for medical advice. Always speak with your healthcare provider about possible interactions with all prescription drugs, vitamins, herbs and supplements, and over-the-counter drugs that you are taking.

Phentermine warnings

This drug comes with several warnings.

Allergy warning

Phentermine can cause a severe allergic reaction. Symptoms can include:

  • trouble breathing
  • swelling of your throat or tongue

If you develop these symptoms, call 911 or go to the nearest emergency room.

Don’t take this drug again if you’ve ever had an allergic reaction to it or phendimetrazine tartrate. Taking it again could be fatal (cause death).

Alcohol interaction warning

The use of drinks that contain alcohol can increase your risk of side effects from phentermine. These side effects can include dizziness, feeling nervous and excitable, headache, hard stools or diarrhea, dry mouth, and not being able to sleep. They can also include decreased interest in sex or change in ability to have sex.

If you drink alcohol, talk to your doctor. You may need to be more closely monitored for side effects.

Warnings for people with certain health conditions

For people with history of primary pulmonary hypertension (PPH): You shouldn’t take phentermine. Phentermine may make your PPH worse.

For people with history of heart disease: You shouldn’t take this drug if you have a history of heart problems. These include stroke, arrhythmia (irregular heartbeat), heart failure, coronary artery disease, valve disease, and uncontrolled high blood pressure. Phentermine may cause a serious increase in your blood pressure. This may make your heart work harder. The extra stress on your heart may make your heart disease worse.

For people with history of hyperthyroidism: Tell your doctor if you have hyperthyroidism (an overactive thyroid). Phentermine may increase your thyroid activity. This may raise your thyroid levels even further.

For people with glaucoma: This drug may increase your eye pressure even more. This may cause permanent damage to your vision. Tell your doctor if you have glaucoma.

For people with diabetes: You may be able to control your diabetes better as you lose weight while you’re on this drug. Your doctor may lower the dosage of your diabetes drugs.

For people with a history of drug misuse: This drug may be habit-forming. You shouldn’t take this medication long-term. Tell your doctor if you have a history of drug or alcohol misuse.

For people with agitation: This drug may cause restlessness and anxiety and may make your agitation worse. You shouldn’t take phentermine if you’re agitated.

Warnings for other groups

For pregnant women: Phentermine is a category X pregnancy drug. That means two things:

  1. Category X drugs should never be used during pregnancy.
  2. Women of childbearing age should use reliable birth control while taking this drug.

Stop taking phentermine and call your doctor right away if you become pregnant while taking this drug.

When to call the doctor

  • Call your doctor if you’re not losing weight, even though you’re exercising, eating a low-calorie diet, and taking this drug. Keep in mind that it may take a few weeks for this drug to work.

For women who are breastfeeding: Phentermine may pass into breast milk and may cause side effects in a child who is breastfed. Talk to your doctor if you breastfeed your child. You may need to decide whether to stop breastfeeding or stop taking this medication.

For seniors: The kidneys of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, a higher amount of a drug stays in your body for a longer time. This raises your risk of side effects.

For children: This medication hasn’t been studied in children. It shouldn’t be used in people younger than 16 years.

How to take phentermine

All possible dosages and drug forms may not be included here. Your dosage, drug form, and how often you take the drug will depend on:

  • your age
  • the condition being treated
  • how severe your condition is
  • other medical conditions you have
  • how you react to the first dose

Drug forms and strengths

Generic: Phentermine

  • Form: oral capsule
  • Strengths: 15 mg, 30 mg, 37.5 mg

Brand: Adipex-P

  • Form: oral capsule
  • Strengths: 37.5 mg

Dosage for obesity

Adult dosage (ages 18–64 years)

  • Adipex-P and generic phentermine 37.5-mg capsule:
    • Typical starting dosage: One 37.5-mg capsule each day. Take it in the morning before breakfast, or 1–2 hours after breakfast.
    • Maximum dosage: 37.5 mg per day
  • Phentermine 15-mg and 30-mg capsules:
    • Typical starting dosage: One 15-mg or 30-mg capsule each day. Take it in the morning, 2 hours after breakfast.
    • Dosage increases: If your doctor started you on 15-mg dose, they may increase your dose based on your weight loss and how well you tolerate the drug.
    • Maximum dosage: 30 mg per day.

Child dosage (age 16–17 years)

  • Adipex-P and generic phentermine 37.5-mg capsule:
    • Typical starting dosage: One 37.5-mg capsule each day. Take it in the morning before breakfast, or 1–2 hours after breakfast.
    • Maximum dosage: 37.5 mg per day.
  • Phentermine 15-mg and 30-mg capsules:
    • Typical starting dosage: One 15-mg or 30-mg capsule each day. Take it in the morning, 2 hours after breakfast.
    • Dosage increases: If your doctor started you on a 15-mg dose, they may increase your dose based on your weight loss and how well you tolerate the drug.
    • Maximum dosage: 30 mg per day.

Child dosage (ages 0–16 years)

It hasn’t been confirmed that this drug is safe and effective for use in children younger than 16 years. It shouldn’t be used in children in this age range.

Senior dosage (ages 65 years and older)

The kidneys of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, a higher amount of a drug stays in your body for a longer time. This raises your risk of side effects.

Your doctor may start you on a lowered dosage or a different dosing schedule. This can help keep levels of this drug from building up too much in your body.

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs affect each person differently, we cannot guarantee that this list includes all possible dosages. This information is not a substitute for medical advice. Always speak with your doctor or pharmacist about dosages that are right for you.

Take as directed

Phentermine oral capsule is used for short-term treatment. It comes with serious risks if you don’t take it as prescribed.

If you stop taking the drug or don’t take it at all: You may not lose weight. If you take this drug for a long period of time and stop it suddenly, you may have extreme fatigue and depression.

If you miss doses or don’t take the drug on schedule: Your medication may not work as well or may stop working completely. For this drug to work well, a certain amount needs to be in your body at all times.

If you take too much: You could have dangerous levels of the drug in your body. Symptoms of an overdose of this drug can include:

  • restlessness
  • tremor
  • confusion
  • fast breathing
  • hallucinations (seeing or hearing something that isn’t there)
  • arrhythmia (irregular heartbeat)
  • hypertension (high blood pressure)
  • heart palpitations (fast, fluttering heartbeat)
  • nausea
  • vomiting
  • diarrhea

If you think you’ve taken too much of this drug, call your doctor or local poison control center. If your symptoms are severe, call 911 or go to the nearest emergency room right away.

What to do if you miss a dose: Take your dose as soon as you remember. But if you remember just a few hours before your next scheduled dose, take only one dose. Never try to catch up by taking two doses at once. This could result in dangerous side effects.

How to tell if the drug is working: You should lose weight.

Important considerations for taking phentermine

Keep these considerations in mind if your doctor prescribes phentermine for you.

General

  • You can take phentermine with or without food.
  • Take phentermine once per day in the morning. Taking phentermine late in the day may cause trouble sleeping.
  • Don’t cut or crush the oral capsule.

Storage

  • Store phentermine at room temperature. Keep it at a temperature between 68°F and 77°F (20°C and 25°C).
  • Keep this drug away from light.
  • Don’t store this medication in moist or damp areas, such as bathrooms.

Refills

A prescription for this medication is refillable. You should not need a new prescription for this medication to be refilled. Your doctor will write the number of refills authorized on your prescription.

Travel

When traveling with your medication:

  • Always carry your medication with you. When flying, never put it into a checked bag. Keep it in your carry-on bag.
  • Don’t worry about airport X-ray machines. They can’t harm your medication.
  • You may need to show airport staff the pharmacy label for your medication. Always carry the original prescription-labeled container with you.
  • Don’t put this medication in your car’s glove compartment or leave it in the car. Be sure to avoid doing this when the weather is very hot or very cold.

Self-management

Your doctor will have you participate in an exercise program and follow a reduced-calorie diet while you’re taking this drug. This should help you lose weight. Your doctor may also ask you to keep a journal of your food intake, exercise, and weight. This will help track your weight loss progress.

Clinical monitoring

Your weight and waist circumference will be monitored while you’re taking this drug. This will tell you and your doctor if the medication is working to help you lose weight.

During your treatment, you and your doctor should monitor certain health issues. This can help make sure you stay safe while you take this drug. These issues include blood pressure.

Your diet

You should follow a reduced-calorie diet as directed by your doctor while taking this drug. This should help you lose weight.

Availability

Not every pharmacy stocks this drug. When filling your prescription, be sure to call ahead to make sure your pharmacy carries it.

Prior authorization

Many insurance companies require a prior authorization for this drug. This means your doctor will need to get approval from your insurance company before your insurance company will pay for the prescription.

Are there any alternatives?

There are other drugs available to treat your condition. Some may be better suited for you than others. Talk to your doctor about other drug options that may work for you.

Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up-to-date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or other healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.