Phentermine Side Effects

Side Effects

Phentermine is the most-prescribed weight loss medication in the United States, but it’s not the perfect pill ( 1 ).

The popular stimulant produces powerful appetite-suppressing and energy-boosting effects which, when used in combination with a healthy diet and regular physical activity, encourage weight loss.

However, in addition to these desirable reactions, phentermine also produces a wide range of unpleasant side effects. The severity of these reactions ranges from mild to life-threatening, so it’s important to understand the risks before undergoing treatment with this medication.

Side Effects of Phentermine

Woman suffering from phentermine side effects, including headache
Many patients deal with phentermine side effects, including headache

Most patients experience at least one phentermine side effect while undergoing treatment. Dry mouth (xerostomia) and trouble sleeping (insomnia) are the most common reactions, but each patient is different.

Severe Side Effects

Phentermine side effect dangers are primarily related to a handful of serious, potentially life-threatening reactions to this medication. Primary pulmonary hypertension (PPH) and heart valve irregularities are two very serious phentermine side effects.

These complications were primarily observed in a group of patients that took the once-popular combination of phentermine and fenfluramine (colloquially referred to as “phen-fen”) in the mid-1990s ( 2 ). While fenfluramine was ultimately deemed responsible for these serious complications, phentermine’s potentially-contributory role could never be ruled out. As a result, these reactions are still listed on all phentermine labels. Click here to read more about phentermine heart problems.

Other serious phentermine side effects relate to mood changes, metabolic imbalances, drug reactions, and allergies.

Symptoms of severe phentermine side effects include ( 3 , 4 ):

  • Chest pain
  • Pounding or irregular heartbeat, or fluttering in your chest
  • Lightheadedness or faintness
  • Shortness of breath, even with only mild exertion
  • Swelling in your legs, ankles, or feet (edema)
  • Slurred speech or weakness on one side of your body
  • Unusual thoughts or behavior, such as confusion, irritability, phen rage, or hallucinations
  • Extreme happiness or sadness (euphoria or phentermine depression)

Any indicators of dangerously-high blood pressure, such as severe headache, blurred vision, buzzing in your ears, anxiety, confusion, chest pain, shortness of breath, uneven heartbeats, or seizure.

Call 911 (emergency medical services) or go to a hospital right away if you notice any of these symptoms while taking phentermine.


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Note that this is not a complete list of side effects. When in doubt, always contact your doctor.


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Common Side Effects

Thankfully, serious side effects are relatively rare. The majority of phentermine users experience reactions that are bothersome, but not typically considered dangerous.

Common phentermine side effects include ( 5 , 6 , 7 , 8 ):

The most common phentermine side effects are dry mouth and trouble sleeping ( 9 ). To that end, a recent online poll of over 2100 phentermine users found that more than 4 in 5 had at least one instance of dry mouth while taking this medication ( 10 ).

Results of a 2019 poll on users' most common phentermine side effects
Results of a 2019 poll on users’ most common phentermine side effects

Side Effects of Qsymia (Phentermine and Topiramate)

Pairing phentermine and topiramate – either as two separate pills or in the popular combined pill, Qsymia – carries an additional risk.

Serious side effects of Qsymia include ( 11 ):

  • Birth defects (cleft lip/palate), if a woman gets pregnant while taking this medication​
  • Increased heart rate or blood pressure​
  • Suicidal thoughts or actions
  • Serious eye problems (secondary angle closure glaucoma)​
  • Increased acid in the bloodstream (metabolic acidosis)​
  • Problems with memory, speech, or concentration​
  • Seizures when medication is stopped suddenly​
  • Kidney stones
  • Decreased sweating/increased body temperature (fever)​
  • Low blood sugar (hypoglycemia) in patients with type 2 diabetes​
  • Low blood sugar (hypoglycemia) in patients with type 2 diabetes​
  • General central nervous system (CNS) side effects​

Do Phentermine Side Effects Go Away?

Woman relieved to be free of phentermine side effects
Phentermine side effects eventually go away for most users

Yes – phentermine side effects are usually worst during the first days or week of treatment.

In most patients, the body starts to return to normal with continued use of the medication. However, certain reactions, such as dry mouth, constipation, or increased heart rate, may last for longer.


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Thankfully, it’s relatively rare for patients to experience phentermine long-term side effects. Hair loss is the most commonly reported long-term reaction, but it is still not listed as an official side effect of the medication.

Contact your doctor if any phentermine side effect does not go away, worsens, or interferes with your daily life.


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How Long Can You Take Phentermine Safely?

Phentermine is a short-term weight loss medication

Phentermine side effects can affect any patient at any time, but some factors make adverse reactions more likely.

Taking phentermine for longer than recommended or prescribed, for example, increases the chances of adverse reactions.

Phentermine is a short-term weight management tool ( 12 ). More specifically, the Food and Drug Administration (FDA) has only approved this medication for “a few weeks” of use, which is generally interpreted to mean no more than 12 weeks at a time ( 12 , 13 ).

It is important to note, however, that this three-month limit is universal for all weight loss drugs approved prior to 1985, related to an old-fashioned belief that lifestyle habits could be modified in 12 weeks’ time ( 1 ). Talk to your doctor if you think you would benefit from a longer-term solution.

How to Combat Side Effects of Phentermine

Staying properly hydrated can help combat some phentermine side effects
Staying properly hydrated can help combat some phentermine side effects

Each phentermine side effect is treated differently depending on the timing, severity, and nature of the reaction. Click on the links above to read more about each individual side effect and how to minimize it.

However, if you are worried about phentermine side effects, make sure to discuss your concerns with the prescribing doctor. He or she can offer advice about strategies to minimize or avoid common reactions to this medication.

Here are a few common suggestions to reduce phentermine side effects:

  • Use the lowest effective dose
  • Drink plenty of water
  • Avoid caffeine and alcohol
  • Include physical activity as permitted and able
  • Tell your doctor about ANY AND ALL other medications and supplements
  • Work with your doctor to optimize the dosage schedule
  • Report any worrisome symptoms to your doctor right away

ALWAYS contact your doctor or seek professional medical attention if you’re concerned about a phentermine side effect, especially if is not going away, worsening, or impacting your daily life.

  1. 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
  2. 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/full/10.1056/NEJM199712113372414
  3. WebMD. (2019). Phentermine Oral: Uses, Side Effects, Interactions, Pictures, Warnings & Dosing.
  4. CardioSmart: American College of Cardiology. (2011, November 21). Phentermine.
  5. Rueda-Clausen, C.F., Padwal, R.S., Sharma, A.M. (2013). New pharmacological approaches for obesity management. Nature Reviews Endocrinology, 9(8):467-478.
  6. Velazquez, A. & Apovian, C.M. (2018). Updates on obesity pharmacotherapy. Annals of the New York Academy of Sciences, 1411(1):106-11.
  7. Kim KK, Cho HJ, Kang HC, Youn BB, Lee KR. (2006). Effects on weight reduction and safety of short-term phentermine administration in Korean obese people. Yonsei Medical Journal, 47(5):614-625. doi: 10.3349/ymj.2006.47.5.614
  8. Alexander, J., Cheng Y.H., Choudhary, J., Dinesh, A. (2011). Phentermine (Duromine) precipitated psychosis. Australian and New Zealand Journal of Psychiatry, 45(8):684-685.
  9. Derosa, G., & Maffioli, P. (2012). Anti-obesity drugs: A review about their effects and their safety. Expert Opinion on Drug Safety, 11(3), 459-471. doi:10.1517/14740338.2012.675326
  10. Members of “Losing Weight with Phentermine” Support Group on Facebook & Forum. (2019, March 13). [User Report of Common Phentermine Side Effects]. Unpublished raw data.
  11. VIVUS, Inc. (2017). Medication Guide.
  12. U.S. Food and Drug Administration. (2012). Adipex-P (phentermine hydrochloride) capsules label [Brochure].
  13. Buchwald, H., Cowan, G. S., & Pories, W. J. (2007). Surgical management of obesity. Philadelphia, PA: Elsevier.