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What is Phentermine?

Dr. Karen Vieira Last Updated: February 21, 2020

Phentermine an amphetamine-like stimulant used for weight loss. It stimulates the central nervous system to induce a “fight or flight”-like reaction, which causes increased heart rate, sweating, lack of appetite and extra energy ( 1 , 2 ).

chemical structure drawings: phentermine vs. amphetamine
A methyl group differentiates phentermine from amphetamine

Medical Use

This drug’s primary use is as a prescription appetite suppressant. In the United States and Latin America, most phentermine-based weight loss pills contain phentermine hydrochloride as their main ingredient. However, phentermine resin-based pills are still available in other parts of the world.

First approved in 1959, phentermine is the most-prescribed weight loss medication in the US ( 3 ).

Who Takes It

Phentermine is a class IV controlled substance, so it is only available under a doctor’s prescription ( 4 ).

This weight loss medication is an option for patients with a body mass index (BMI) greater than or equal to 30 kg/m2, which puts them in the “obese” range. Phentermine is also available to individuals with a BMI of 27-29.9 kg/m2 plus a weight-related condition like type 2 diabetes, high cholesterol or controlled hypertension ( 1 ).

However, this medication is not appropriate for all overweight and obese patients. There are many contraindications for phentermine use, including, but not limited to: history of cardiovascular disease, history of addiction, history of agitated states or current/recent use of certain medications or supplements ( 5 ).

doctor taking a man's blood pressure


Phentermine weight loss pills are available in both tablet and capsule form, in doses ranging from 8 – 37.5 milligrams.

The lowest dose tablets (Lomaira 8mg) can be taken up to three times per day, while higher dose tablets and capsules can only be taken 1-2 times per day. Exact phentermine dosage and schedule varies by individual.

8 mg tablet15-18.75 mg capsule30-37.5 mg
capsule or tablet

The maximum dose of phentermine hydrochloride is 37.5 mg per day.

Adverse Reactions

Phentermine is a highly-effective weight loss medication, but it also has a long list of potential side effects.

Serious reactions to phentermine, which may be symptoms of a potentially-fatal heart or lung problem, include ( 5 , 6 ):

  • Chest pain
  • Irregular (fluttering) or pounding heart rate
  • Shortness of breath
  • Lightheadedness or fainting
  • Swelling of the ankles, legs or feet
  • Stroke-like symptoms, such as slurred speech or one-sided weakness
  • Extreme happiness or sadness

However, most people experience only side effects. The most-commonly reported reactions are dry mouthinsomnia, constipation and headache (7).

Most patients report fewer side effects as the treatment progresses. Lower dose pills or alternate schedules can also help alleviate the bothersome reactions faster.


In spite of the growing necessity for pharmaceutical weight loss options, many doctors and potential patients remain skeptical of phentermine due to its troubled past.

During the 1980-90s, doctors commonly prescribed an off-label combination of fenfluramine (fen) and phentermine (phen) for weight loss ( 8 ). However, in 1997, the Mayo Clinic reported that “fen-phen”caused heart valve irregularities and primary pulmonary hypertension in some patients. Fenfluramine, the likely culprit, was withdrawn from the market soon after ( 9 , 10 ).

Phentermine has remained available with an added warning on the package insert ( 1 ).

  1. U.S. Food and Drug Administration. (2012). Adipex-P (phentermine hydrochloride) capsules label [Brochure].
  2. National Center for Biotechnology Information. PubChem Compound Database: CID=4771 (Phentermine).
  3. Hendricks, E. J., Rothman, R. B., & Greenway, F. L. (2009). How Physician Obesity Specialists Use Drugs to Treat Obesity. Obesity, 17(9), 1730-1735. doi:10.1038/oby.2009.69
  4. Rueda-Clausen, C.F., Padwal, R.S., Sharma, A.M. (2013). New pharmacological approaches for obesity management. Nature Reviews Endocrinology, 9(8):467-478.
  5. WebMD. (2019). Phentermine Oral: Uses, Side Effects, Interactions, Pictures, Warnings & Dosing.
  6. CardioSmart: American College of Cardiology. (2011, November 21). Phentermine.
  7. Members of “Losing Weight with Phentermine” Support Group on Facebook & Phentermine.com Forum. (2019, March 13). [User Report of Common Phentermine Side Effects]. Unpublished raw data.
  8. Weintraub M, Hasday JD, Mushlin AI, Lockwood DH. A double-blind clinical trial in weight control. Use of fenfluramine and phentermine alone and in combination. Arch Intern Med. 1984;144(6):1143-1148.
  9. Connolly, H. M., Crary, J. L., McGoon, M. D., Hensrud, D. D., Edwards, B. S., Edwards, W. D., & Schraff, H. V. (1997). Valvular Heart Disease Associated with Fenfluramine–Phentermine. New England Journal of Medicine, 337(24), 1772-1776. doi:10.1056/nejm199712113372414
  10. Centers for Disease Control and Prevention. (1997). Cardiac Valvulopathy Associated With Exposure to Fenfluramine or Dexfenfluramine: US Department of Health and Human Services Interim Public Health Recommendations, November 1997. JAMA: The Journal of the American Medical Association, 278(21), 1729. doi:10.1001/jama.1997.03550210025016
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