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What is phentermine?

Phentermine is a prescription drug that can help lower your body weight.

Generic phentermine is available in two forms:

  • Capsule (15mg, 30mg, and 37.5mg)

  • Tablet (8mg and 37.5mg)

Phentermine belongs to the sympathomimetic amine anorectic class of drugs, commonly known as stimulant drugs. It works by stimulating the release of norepinephrine, serotonin, and dopamine in the brain to decrease your appetite.

It is approved for use as a short-term (12-week) treatment, but doctors prescribe it off-label for longer use in some cases. Doctors sometimes prescribe phentermine for intermittent use.

Phentermine is a controlled medicine and is listed as a Schedule IV drug. This means that it has a low probability of misuse, but you should not take it for longer than your doctor recommends.

Phentermine: brand names

Phentermine is also sold under different brand names, including:

  • Adipex-P (phentermine hydrochloride): available in capsule and tablet form

  • Lomaira: (phentermine hydrochloride) available as a tablet

  • Ionamin (phentermine resin): available as a capsule

  • Suprenza (phentermine hydrochloride): available as orally disintegrating tablets

  • Qsymia (phentermine hydrochloride and topiramate): available as a capsule

Note that this article will focus on generic phentermine. Ask your doctor or pharmacist for more information about any of the formulations listed above.

What is phentermine used to treat?

Doctors prescribe phentermine to people who are obese to help them lose weight. It should be used together with other measures, like eating a healthy diet and exercising regularly.

The drug is only recommended for the following people with obesity:

  • People with a BMI of 30kg/m2 and above

  • People with a BMI of 27kg/m2 and the risk of high blood pressure, cardiovascular events, diabetes, or high cholesterol

Your doctor may prescribe phentermine if lifestyle changes and other measures haven’t helped you lose weight.

How do you take phentermine?

Take phentermine as prescribed by your doctor. Don’t start or stop taking phentermine without your doctor’s advice. Don’t adjust your dosage on your own.

Take the medication whole; do not crush or chew the tablet or capsule. Some tablets are scored to be split in half but only do so under your doctor’s recommendations.

Phentermine is not recommended for patients younger than 16 years of age.

You should also take steps to improve your diet and exercise when taking phentermine.

Dosage for treating obesity

The standard dosage of generic phentermine for adults with obesity in tablet or capsule form is 37.5mg daily¹, taken an hour before eating breakfast or one to two hours after breakfast.

Your doctor may recommend taking two separate 18.75mg doses daily. Some people find a half dose of 18.75mg is effective.

Ask your doctor or pharmacist what dosage to take if you have been prescribed phentermine in a different format.

Seeing results

The results you see will depend on several factors, including your initial BMI, sex, muscle mass, exercise routine, diet, and dosage.

You could expect to start seeing results within the first two to three weeks. Based on these results, your doctor will reevaluate your weight-loss plan to determine if you should continue on the same dose, take a higher or lower dose, or stop taking phentermine.

A postmarketing study² of phentermine found that 45% of trial participants lost more than or equal to 5% of their body weight after three months.

Another study³ reported that participants who used phentermine for more than 12 months had lost 7.4% more weight at 24 months than those who used the drug for less than three months.

Not everyone responds to phentermine. If you don’t see any results after three months of treatment, your doctor may recommend stopping phentermine treatment and trying something else.

Potential side effects of phentermine

Phentermine can cause side effects.

Common side effects of phentermine are:

  • Dizziness

  • Headache

  • Dry mouth

  • Constipation

  • Increased and/or decreased libido

  • Trouble sleeping

  • Nausea

  • Feeling restless

These symptoms usually go away on their own. However, if you experience these symptoms for an extended period and they worsen, call your doctor for medical advice.

Severe side effects of phentermine include:

  • Seizures

  • Signs of stroke (including weakness on only one side of the body, severe headache, trouble speaking, and blurred vision)

  • Elevation of blood pressure

  • Hallucinations

  • Heart palpitations

  • Suicidal thoughts

Rare but sometimes fatal side effects of phentermine include primary pulmonary hypertension (PPH). PPH may be more of a risk if you combine phentermine with fenfluramine or dexfenfluramine. Common initial signs of PPH include:

  • Shortness of breath (dyspnea) — you might notice exercise becomes more challenging

  • Chest pain

  • Fainting

  • Fluid build-up (edema) in the lower legs

Another rare but serious side effect of phentermine is valvular heart disease. This may be more likely if you combine phentermine with fenfluramine or dexfenfluramine.

Long-term use of phentermine

Phentermine is only approved by the FDA for short-term use, although some doctors prescribe the drug off-label for long-term treatment.

Long-term use of phentermine is reported in some individuals to cause psychological and physical dependence, particularly with higher doses. Withdrawal symptoms are also likely to occur when using phentermine for a long time.

Taking phentermine for a long period may also increase your risk of heart valve problems and pulmonary hypertension, especially when combined with other appetite-suppressing medications.

Missed doses

Take a missed dose as soon as you remember. If it is already late in the day, skip the missed dose and take it the next day.

Do not take two doses of phentermine at the same time.


The following symptoms may indicate you have taken too much phentermine:

  • Hallucinations

  • Panic

  • Nausea

  • Vomiting

  • Stomach cramps

  • Diarrhea

  • Extreme restlessness

  • Feeling tired

  • Feeling depressed

  • Weak pulse

  • Irregular heartbeat

  • Slow breathing

  • Seizure

An overdose of phentermine is known to be fatal. Seek urgent medical attention if you think you or someone else has taken too much.

Allergy information

Phentermine contains inactive ingredients that may cause allergic reactions. Symptoms of an allergic reaction include the following:

  • Trouble breathing

  • Swelling of the tongue and throat

  • Severe dizziness

  • Rash

  • Itching

Contact your doctor if you think you are having an allergic reaction to phentermine. You may need to take a different medication instead.

Taking phentermine again after experiencing an allergic reaction may cause serious and even fatal effects.

What to discuss with your doctor before taking phentermine

Before you start using phentermine, discuss the following with your doctor:

  • Your allergies: Inform your doctor if you are allergic to phentermine or its ingredients. Consult the product label or ask your pharmacist to find out the ingredients in your medication.

  • Your past and present medical conditions: Tell your doctor about any other health conditions, such as overactive thyroid, glaucoma, extreme agitation or nervousness, drug abuse history, high blood pressure, or history of heart, renal, or liver disease.

  • Other medications, vitamins, or supplements you are currently taking, including those you intend to start taking while undergoing phentermine treatment. Other medications you take may interact with phentermine.

  • Pregnancy and breastfeeding: If you are pregnant, planning to become pregnant, or are currently breastfeeding, tell your doctor. Phentermine is not recommended for pregnant women.

Stopping phentermine

Do not suddenly stop using phentermine.

Suddenly stopping phentermine treatment can cause seizures. It can also cause withdrawal symptoms.

Withdrawal symptoms include:

Ask your doctor for instructions if you need to stop using phentermine.

Phentermine and pregnancy

Taking phentermine while pregnant is not recommended.

Phentermine is categorized as a pregnancy category X⁴ drug, meaning the risks outweigh the potential benefits. Weight loss does not benefit pregnant women as overall weight gain is recommended. Taking this drug during pregnancy could harm the fetus.


It is not known⁵ whether phentermine can pass into breast milk, so talk to your doctor about the potential risks. Phentermine is an amphetamine-like drug, and amphetamines have been known to enter breast milk.

Interactions with other drugs

Do not take phentermine if you have taken monoamine oxidase inhibitors (MAOIs) in the last two weeks. These medications include:

  • Phenelzine (Nardil)

  • Rasagiline (Azilect)

  • Selegiline (Eldepryl or Zelapar)

  • Linezolid (Zyvox)

  • Isocarboxazid (Marplan)

  • Tranylcypromine (Parnate)

  • Selegiline skin patch

Do not combine phentermine with other weight-loss medications, including:

  • Fenfluramine (Phen-Fen)

  • Dexfenfluramine (Redux)

Phentermine can also cause harmful effects when taken with the following drugs:

Phentermine has mild interactions with the following supplements:

  • Amantadine

  • American ginseng

  • Eucalyptus

  • Guarana

  • Sage

  • Yerba mate

  • Desmopressin

You should tell your doctor if you have had a methylene blue test injection in the past two weeks.

Avoid drinking alcohol while undergoing phentermine treatment.

This is not a complete list of drugs and medications that interact with phentermine. Let your doctor know about all medications you are taking to avoid potentially harmful complications.

Clinical trial history

Phentermine is the most commonly prescribed weight-loss medication in the US⁶. Despite this, there are few controlled trials of phentermine monotherapy for six months or more.

Double-blind clinical trial (1984)

The longest clinical trial⁷ carried out with phentermine took about 36 weeks. It consisted of 108 women with obesity divided into three groups:

  1. Continuous treatment with 30mg of phentermine each day

  2. Intermittent (every other month) treatment with 30mg of phentermine each day

  3. Placebo

Group 1 participants lost an average of 12.2kg of weight.

Group 2 participants lost an average of 13kg of weight.

Group 3 participants lost an average of 4.8kg of weight.

The efficacy is likely overstated since attrition was 41%, and the results presented were only for those who completed the trial.

Meta-analysis (2002)

A 2002 meta-analysis⁸ of the pharmacotherapy for obesity assessed six studies published between 1975 and 1999.

Researchers found phentermine, when used in addition to lifestyle interventions, led to an increase in weight loss. This effect was described as “statistically significant but modest⁹.” The dosage assessed was between 15mg and 30mg per day. Compared to placebo, patients lost an additional 3.6kg on average.

Drug approval history

1959: Phentermine is first approved by the FDA as Ionamin (capsule).

1980: Adipex-P is approved by the FDA.

2011: The FDA approves a new formulation of phentermine — Suprenza (orally disintegrating tablet).

2012: Qsymia (phentermine and topiramate) extended-release capsule is approved by the FDA.

2016: Lomaira (phentermine oral tablet) is approved by the FDA.

Tips and advice for taking phentermine

Follow these tips to help you take phentermine safely and see the best results:

  • Phentermine is known to cause insomnia, so avoid taking it in the evening.

  • Phentermine can cause dizziness. Don’t drive or undertake specific activities until you know how the drug affects you.

  • Alcohol can worsen the side effects of phentermine, so avoiding alcohol while taking this medication is recommended.

  • Phentermine is a controlled substance. Improper use may cause addiction. Use phentermine according to your prescription.

  • Keep phentermine in a cool dry place, out of reach of children.

  • Though phentermine has some serious side effects, your doctor assessed your condition and prescribed you this medication because the benefits outweigh the risks.

  • Do not take other medications for weight loss when using phentermine. It may cause serious interactions.

  • It is recommended to check your blood pressure regularly as phentermine is known to cause high blood pressure.

  • Checking your blood sugar regularly is recommended if you have diabetes. Your diabetes medication may require adjustments when using phentermine.

  • If you are having any type of surgery, inform the medical/dental team that you are using phentermine.

  • Try to avoid heat as much as possible. Phentermine is known to possibly prevent the body from sweating, and make it harder for the body to cool down. Drink plenty of fluids while taking phentermine.

  1. Phentermine hydrochloride tablet, Phentermine hydrochloride capsule | Daily Med

  2. Postmarketing surveillance study of the efficacy and safety of phentermine in patients with obesity (2013)

  3. Safety and effectiveness of longer-term phentermine use: Clinical outcomes from an electronic health record cohort (2019)

  4. Adipex-p (phentermine hydrochloride) capsules label (2012)

  5. Phentermine pregnancy and breastfeeding warnings |

  6. Anti-obesity medication use in 2.2 million adults across 8 large healthcare organizations: 2009-2015 (2020)

  7. A double-blind clinical trial in weight control: Use of fenfluramine and phentermine alone and in combination (1984)

  8. Pharmacotherapy for obesity: a quantitative analysis of four decades of published randomized clinical trials (2002)

  9. Meta-analysis: Pharmacologic treatment of obesity (2005)

Other sources:

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.


Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.