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Long-term use of weight-loss drug may be safe and effective

Short-term use of the weight-loss drug phentermine is approved by the US FDA. Researchers conducted an analysis to determine if its long-term use is also safe and effective.


Lifestyle changes can help people with obesity lose weight. These interventions result in only 5-10% weight loss after six months. Also, about one-third of individuals will not respond to lifestyle changes. Many people regain the weight that was once lost. Weight-loss drugs can be an interesting strategy for obese people having difficulty losing weight.

Phentermine suppresses appetite

Phentermine is a weight-loss drug that suppresses the appetite. The United States Food and Drug Administration (FDA) approved phentermine as a weight-loss drug almost 60 years ago. Most studies on phentermine for weight-loss have limited treatment durations to less than 12 weeks. Concerns regarding the safety of long-term phentermine treatment have limited the use of this weight-loss drug to only a few weeks.

Researchers from the Department of Epidemiology and Prevention at the Wake Forest School of Medicine in North Carolina designed a study to compare short-term and long-term use of phentermine. The primary focus of the study was the difference in weight loss and cardiovascular disease and death rates in the short-term and long-term treatment groups. The results of the study were published in Obesity.

Researchers formed four groups of patients. The first two groups included people taking phentermine continuously. The short-term use group used phentermine continuously for 16 weeks or less. The long-term group took phentermine for more than 365 consecutive days.

Two other groups of patients were created to capture data from people taking phentermine intermittently. One group included individuals with two or more episodes of phentermine therapy in which neither episode lasted more than 16 weeks. This was the short-term intermittent group. The last group included individuals with two or more episodes of taking phentermine, but at least one episode was longer than 16 weeks. The last group was named medium-term intermittent.

Long-term use of phentermine was associated with more weight loss

The results of this study demonstrate that long-term treatment with phentermine was associated with greater weight loss. At six, 12, and 24 months after starting phentermine weight loss was greater in the long-term, short-term intermittent, and medium-term intermittent groups compared with the FDA approved short-term therapy group.

The amount of weight loss in each group varied at each follow-up time. At six months, the short-term intermittent group lost more weight than the short-term continuous group. However, the medium-term intermittent group lost more weight than all other groups at the 6-month and 12-month follow-up time. After two years from starting the weight-loss drug, the long-term continuous group experienced the greatest weight loss among the other groups.

The safety of taking this drug for longer treatment periods was also evaluated. The most significant concern about using phentermine for more than three months was the risk of cardiovascular side effects and death.

No apparent increase in heart disease risk or death rates with long-term use

A slight increase in heart rate was observed among phentermine users, but heart rates went back to normal after stopping the weight-loss drug. Systolic blood pressure was actually lower in individuals using phentermine for long-term treatment. This might be explained by the effect of losing weight on blood pressure and not the weight-loss drug itself. The researchers also did not notice any increase in cardiovascular disease risk or any increase in death rates among long-term phentermine users.

Researchers did not know if participants were involved in other weight-loss programs

Several limitations of this study require consideration. First of all, the researchers did not note any other health care provider interventions for weight loss. There is no way for the researchers to know if long-term users of phentermine were also involved in other programs that could contribute to weight loss. Also, the group of long-term continuous phentermine users was small compared with the other groups. Further studies are required to test the safety and efficacy of phentermine in a larger group of people and for longer treatment times.

Phentermine may be a reasonable option with low heart disease risk

Many people are searching for a safe weight-loss drug that results in permanent weight loss. Despite the obvious limitations in this study, phentermine seems to be safe and effective when used for treatment times that surpass three months. Over the longer follow-up time, there was greater weight loss without increases in cardiovascular disease risk or death. Although further studies are required to confirm these long-term results, phentermine seems to be a reasonable option for people with low cardiovascular disease risk.

Written by Jessica Caporuscio, PharmD


  1. Lewis KH, Fischer H, Ard J, et al. Safety and Effectiveness of Longer-Term Phentermine Use: Clinical Outcomes from an Electronic Health Record Cohort. Obesity (Silver Spring). 2019
  2. Phentermine: MedlinePlus Drug Information. (n.d.). Retrieved April 3, 2019, from
Jessica Caporuscio PharmD
Jessica Caporuscio PharmD
Jessica received her Doctorate of Pharmacy from the University of Montreal in July 2014 and she has been working as a community pharmacist in the Greater Toronto Area since March 2015. She has a passion to communicate medical and drug information to her patients and other healthcare professionals. Jessica is also a marathoner and an Ironman triathlete. In her spare time, she is in her kitchen creating healthy recipes for her family.


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