Phentermine is the most popular weight loss medication in the United States, but it is not the perfect pill .
This medication, which is used in combination with healthy diet and regular physical activity, stimulates the central nervous system to produce powerful appetite suppressing and energy boosting effects to encourage weight loss.
However, in addition to these desired and intended responses, phentermine also has a wide range of adverse side effects. The severity of these potential reactions ranges from mild to life-threatening, so it is important to be informed about possible phentermine side effects before undergoing treatment with this medication.
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.
Most of the phentermine side effects dangers are related to a handful of serious, potentially life-threatening reactions to this medication. Primary pulmonary hypertension (PPH) and heart valve irregularities are two of the most serious potential phentermine side effects.
These complications were observed in a group of patients that took the once-popular combination of phentermine and fenfluramine (colloquially referred to as “phen-fen”) in mid-1990s . While fenfluramine was ultimately deemed responsible for these complications, phentermine’s potentially-contributory role could never be definitively ruled-out. Click here to read more about phentermine heart problems.
Other serious side effects are largely related to possible mood changes, metabolic imbalances, drug reactions and allergies.
Thankfully, the above side effects prove relatively rare in patients taking phentermine weight loss pills. Most users experience reactions that are bothersome, but not typically considered dangerous.
The most common phentermine side effects are dry mouth (xerostomia) and trouble sleeping (insomnia) . A recent online poll of over 2100 phentermine users corroborates this finding, with more than 4 in 5 respondents reporting at least one instance of dry mouth during their weight loss journey .
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:
Yes – phentermine side effects are usually worst during the first days or week of treatment.
In most patients, the body reacts most while it is adjusting to the stimulant, but then 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, depending on the individual.
Phentermine long term side effects remain rare, thankfully. Based on anecdotal evidence, hair loss is the most common lasting reaction to phentermine treatment, however it is not listed as an official side effect of the medication.
Phentermine side effects can affect any patient at any time, but some circumstances make adverse reactions more likely. Taking phentermine for longer than recommended or prescribed is one such circumstance that increases the risk of unwanted side effects during (or after) treatment.
Phentermine is a short-term weight management tool, designed as an adjunct therapy to behavior modification, caloric restriction and exercise . More specifically, the Food and Drug Administration (FDA) only approved this medication for “ a few weeks” of use, which is generally interpreted to mean 12 weeks at a time [12, 13].
This stated restriction is related to concerns about addiction and tolerance, as well as a lack of long-term clinical trials to assess phentermine’s long term side effects.
It is important to note, however, that this 3-month limit is universal for all weight loss drugs approved prior to 1985 given an old-fashioned belief that lifestyle habits could be modified in ≤ 12 weeks . Talk to your doctor if you think you would benefit from a longer-term solution.
Each phentermine side effect is treated differently depending on the timing, severity and nature of the reaction. For more advice on how to reduce side effects of phentermine, click on the links above that lead to more information about each individual side effect.
If you are worried about phentermine side effects, it is also beneficial 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 some of the most common suggestions:
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:10.1056/nejm199712113372414
4. CardioSmart: American College of Cardiology. (2011, November 21). Phentermine.
7. Kim K.K., Cho H.J., Kang H.C., et. al. (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
11. VIVUS, Inc. (2017). Medication Guide.
12. U.S. Food and Drug Administration. (2012). Adipex-P (phentermine hydrochloride) capsules label [Brochure].