Reviewed on 8/4/2022

What Is Phentermine and How Does It Work?

Phentermine is a prescription medication used with a doctor-approved exercise, behavior change, and reduced-calorie diet program for weight loss. It is used by certain overweight people, such as those who are obese or have weight-related medical problems. Losing weight and keeping it off can lessen the many health risks that come with obesity, including heart disease, diabetes, high blood pressure, and a shorter life.

  • It is not known how this medication helps people to lose weight. It may work by decreasing your appetite, increasing the amount of energy used by your body, or by affecting certain parts of the brain. This medication is an appetite suppressant and belongs to a class of drugs called sympathomimetic amines.
  • Phentermine is available under the following different brand names: Adipex P, and Lomaira.

What Are Side Effects Associated with Using Phentermine?

Common side effects of Phentermine include:

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

Serious side effects of Phentermine include:

  • hives.
  • difficult breathing.
  • swelling of your face, lips, tongue, or throat.
  • 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.

Rare side effects of Phentermine include:

  • none

Seek medical care or call 911 at once if you have the following serious side effects:

  • Severe headache, confusion, slurred speech, arm or leg weakness, trouble walking, loss of coordination, feeling unsteady, very stiff muscles, high fever, profuse sweating, or tremors.
  • Serious eye symptoms such as sudden vision loss, blurred vision, tunnel vision, eye pain or swelling, or seeing halos around lights.
  • Serious heart symptoms such as fast, irregular, or pounding heartbeats; fluttering in the chest; shortness of breath; sudden dizziness, lightheartedness, or passing out.

This is not a complete list of side effects and other serious side effects or health problems that may occur because of the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may report side effects or health problems to FDA at 1-800-FDA-1088.


Surprising Causes of Weight Gain See Slideshow

What Are Dosages of Phentermine?

Adult and pediatric dosage

capsule, HCl salt: Schedule IV

  • 15 mg (generics)
  • 30 mg (generics)
  • 37.5 mg (Adipex P, generics)
  • tablet, HCl salt: Schedule IV
  • 8 mg (Lomaira)
  • 37.5 mg (Adipex P, generics)

Weight management

Adult dosage

  • Capsule, tablet (excluding Lomaira): Oral: 15 to 37.5 mg once daily before breakfast or 1 to 2 hours after breakfast; in some cases, may administer 18.75 mg once or twice daily using the tablets.
  • Tablet (Lomaira only): Oral: 8 mg 3 times daily 30 minutes before meals.
  • Orally disintegrating tablet: Oral: 15 to 37.5 mg once daily every morning.

Pediatric dosage

  • Capsules: Administer before breakfast or 1 to 2 hours after breakfast.
  • Tablets:8 mg tablets (Lomaira): Administer 30 minutes before meals. Tablets are scored and may be divided in half.
  • 37.5 mg tablets: Administer before breakfast or 1 to 2 hours after breakfast. Tablets may be divided in half and doses may be given in 2 divided doses.
  • Orally disintegrating tablets (ODT): Administer in the morning, with or without food. With dry hands, place the tablet on the tongue and allow it to dissolve, then swallow with or without water.

Dosage Considerations – Should be Given as Follows

  • See “Dosages”

What Other Drugs Interact with Phentermine?

If your doctor has directed you to use this medication, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider, or pharmacist first.

This document does not contain all possible interactions. Therefore, before using this product, tell your doctor or pharmacist about all the products you use. Keep a list of all your medications with you, and share the list with your doctor and pharmacist. Check with your physician if you have health questions or concerns.

What Are Warnings and Precautions for Phentermine?


  • This medication contains phentermine. Do not take Adipex P or Lomaira if you are allergic to phentermine or any ingredients contained in this drug.
  • Keep out of reach of children. In case of an overdose, get medical help or contact a Poison Control Center immediately.


Effects of Drug Abuse

  • Phentermine may cause withdrawal reactions, especially if it has been used regularly for a long time or in high doses. In such cases, withdrawal symptoms (such as depression, and severe tiredness) may occur if you suddenly stop using this medication. To prevent withdrawal reactions, your doctor may reduce your dose gradually. Consult your doctor or pharmacist for more details, and report any withdrawal reactions right away.
  • Rarely, abnormal drug-seeking behavior (addiction) is possible with this medication. Do not increase your dose, take it more frequently, or use it for a longer time than prescribed. Properly stop the medication when so directed.

Short-Term Effects

  • Phentermine may raise your blood pressure. Check your blood pressure regularly and tell your doctor if the results are high.

Long-Term Effects

  • Phentermine may rarely cause serious (sometimes fatal) lung or heart problems (pulmonary hypertension, heart valve problems). The risk increases with longer use of this medication and use of this drug along with other appetite-suppressant drugs/herbal products. If you notice any of the following unlikely but very serious side effects, stop taking this medication and consult your doctor or pharmacist right away: chest pain, difficulty breathing with exercise, decreased ability to exercise, fainting, swelling of the legs/ankles/feet.


  • Primary pulmonary hypertension and valvular disease have been reported with therapy.
  • Avoid the use of stimulants in patients with serious heart rhythm disturbances, cardiac abnormalities, cardiomyopathies, or other cardiac problems.
  • Caution in hypertension; risk of increase in blood pressure.
  • Patients with diabetes mellitus should use caution with antidiabetic agents like insulin or other hypoglycemic agents; dietary restrictions may lower the requirements for antidiabetic therapy.
  • The drug has not been studied in renal impairment.
  • Use caution in patients with seizure disorders or Tourette syndrome.
  • May impair the ability to operate heavy machinery.
  • Concomitant administration with other weight-loss agents (e.g., selective serotonin reuptake inhibitors [SSRIs], herbal preparations, other over-the-counter products) has not been studied.
  • The elderly are at high risk for myocardial infarction, hypertension, angina, or becoming dependent on phentermine with prolonged use.
  • Discontinue if weight loss has not occurred with 4 weeks of therapy.
  • Not approved for long-term use.
  • Discontinue if tolerance develops, and do not exceed the recommended dosage to overcome tolerance.

Pregnancy and Lactation

  • Do not use phentermine in pregnancy. The risks involved outweigh the potential benefits. Safer alternatives exist. It is unknown if phentermine is excreted in the milk; the use of phentermine is contraindicated while breastfeeding.
  • 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 breastfeed while using phentermine.
Medscape. Phentermine.

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