Phentermine is known for its long list of potential side effects, ranging from relatively-mild (e.g. dry mouth or insomnia) to severe and potentially life-threatening (e.g. heart problems). However, phentermine’s long-term side effects remain largely unknown. This is one significant reason why many doctors only prescribe phentermine as a short-term treatment for obesity.
Phentermine is FDA-approved for short-term use, which is generally interpreted to mean no more than 12 weeks at a time. As a result, most studies have not examined how longer periods of use affect patients and there exists very little research on the long-term side effects of phentermine [1-3].
However, informal anecdotal evidence from online discussion boards suggests that most patients’ side effects dramatically improve or completely disappear after stopping treatment.
The most notable exception to this rule is phentermine hair loss, which can persist for weeks or months after the last dose. This unofficial side effect does not immediately disappear because drug- or stress-induced hair loss occurs as a result of follicle damage early in hair’s growth process . Consequently, it can take months for hair to grow back.
One recent study looked at the long-term side effects of phentermine use in healthy adults.
Published in the April 2019 issue of Obesity, this paper by Lewis et. al examined overall weight loss and cardiovascular risk associated with longer-term phentermine use . More broadly, researchers wanted to know: “What are the long-term side effects of taking phentermine?”
This study looked at nearly 14,000 adult participants with an average body mass index (BMI) of 37.8 kg/m2. Patients were grouped into five different groups based on duration of treatment (ranging from less than 3 months to over 1 year), and then evaluated based on outpatient appointment data collected at 6, 12 and 24 months. Researchers looked at trends in weight, resting heart rate and blood pressure, as well as incidence of adverse cardiovascular events or death.
Results of this study suggest that there is no significant correlation between longer-term phentermine use (up to 24 months) and adverse cardiovascular outcomes. In fact, long-term phentermine users actually experienced a drop in systolic blood pressure over the course of treatment – likely related to their significant weight loss. Some users had a higher resting heart rate while actively taking phentermine, but pulse returned to normal after treatment stopped.
Unsurprisingly, weight loss was more significant in longer-term phentermine users. Still, weight gain after stopping phentermine remained a noteworthy problem in all treatment groups.
While further research is needed to confirm the safety and efficacy of longer-term phentermine use, these findings are a promising stepping stone for proponents of longer-term phentermine monotherapy as a treatment for chronic obesity.
Learn more about phentermine!
1. U.S. Food and Drug Administration. (2012). Adipex-P (phentermine hydrochloride) capsules label [Brochure]. Retrieved from https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/085128s065lbl.pdf
5. Lewis, K. H., Fischer, H., Ard, J., Barton, L., Bessesen, D. H., Daley, M. F., . . . Arterburn, D. E. (2019). Safety and Effectiveness of Longer‐Term Phentermine Use: Clinical Outcomes from an Electronic Health Record Cohort.Obesity, 27(4), 591-602. doi:10.1002/oby.22430