Long-Term Side Effects

Long-Term Side Effects

Phentermine is known for its long list of potential side effects, but these reactions listed tend to occur while a patient is still taking the medication. Phentermine’s long-term side effects, on the other hand, remain largely unknown.

Long-Term Side Effects of Phentermine

desk calendar
Phentermine should only be used for a few weeks at a time

Phentermine is FDA-approved for short-term use, which is generally interpreted to mean no more than 12 weeks at a time. Most studies have not examined how longer periods of use affect patients and there is very little research on the long-term side effects of phentermine ( 1 , 2 , 3 ).

Users’ comments in online discussion boards, however, suggest that most patients’ side effects improve or 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. Hair may take months to grow back because drug- or stress-induced hair loss often occurs as a result of follicle damage early in hair’s growth process, meaning new hair needs to come-in before damage can be erased ( 4 ).

Research on Side Effects of Long-Term Use

doctor listening to young man's heart
Phentermine’s long-term side effects remain largely unknown

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 ( 5 ). 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 assigned to one of to five different groups based on the 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 the 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 their pulse returned to normal after treatment stopped.


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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.

The study did NOT examine the incidence of other potential, long-term phentermine side effects such as dry mouth, insomnia, or mood changes.

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.

Back to All Phentermine Side Effects

  1. U.S. Food and Drug Administration. (2012). Adipex-P (phentermine hydrochloride) capsules label [Brochure].
  2. Buchwald, H., Cowan, G. S., & Pories, W. J. (2007). Surgical management of obesity. Philadelphia, PA: Elsevier.
  3. Velazquez, A. & Apovian, C.M. (2018). Updates on obesity pharmacotherapy.Annals of the New York Academy of Sciences, 1411(1):106-119.
  4. Patel, S., & Tosti, A. (2014). An overview of management of drug-induced hair and nail disorders.Clinical Practice, 11(3), 327-339. doi:10.2217/cpr.14.18
  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