Do Anti-Obesity Medications Reduce Weight Gain After Gastric Bypass?

Bariatric surgery is the most effective treatment for obesity. However, since obesity is a chronic disease, many patients experience weight gain following Roux-en-Y gastric bypass surgery.

A study by Nawfal Istfan published in May 2020 investigated whether anti-obesity drugs could reduce the risk of such weight regain ( 1 ).

Reasons for Post-RYGB Weight Gain

Post-operative weight gain can occur for several reasons ( 2 ).

  1. Increasing your calorie intake
  2. Sedentary lifestyle
  3. Physical changes in the structure of your stomach or intestine have allowed for a bigger appetite and greater absorption of calories.

Although a structural change could mean more surgery, weight regain after bariatric surgery due to a slip in diet or exercise will require lifestyle re-assessment. However, anti-obesity medications, including phentermine, could offer the helping hand you need to address any unwanted pounds.

What Percentage of Gastric Bypass Patients Regain Weight?

Weight regain affects at least 25% of patients following Roux-en-Y gastric bypass surgery (RYGB). It is common to start putting on weight around two years after the surgery ( 3 ).

How to Avoid Weight Regain

While bariatric surgery offers excellent results, long-term modifications are required for weight maintenance once your healthy goal has been achieved. Patients who undergo surgery are advised of the need for lifelong lifestyle changes which include:

  1. Attending all follow-up appointments to monitor any weight regain after a gastric bypass
  2. Carefully following the diet recommended by your nutritionist or dietician ( 4 )
  3. Maintaining high levels of exercise, including walking, swimming, or cycling

Lifetime follow-up by bariatric specialists is expected due to the complex nature of the surgery and its effects on nutrient absorption. Attending regularly is a crucial factor in successful long-term weight loss ( 4 ).

What Are the Benefits of Anti-Obesity Medications After Bariatric Surgery?

Only a limited number of studies have ever taken place to investigate long-term outcomes when using anti-obesity medications after gastric bypass surgery.

As one of the most prominent long-term studies, Istfan and his colleagues analyzed the behavior of 1,196 multi-ethnic patients for 11 years.

During this time, any weight regain was measured against their so-called “nadir weight”, which is the lowest recorded weight for an individual patient following RYGB ( 1 ).

Half of the patients studied regained at least one-third of the weight they had lost post-operatively. Yet, Istfan’s longitudinal study also found that taking phentermine, with or without an additional anti-obesity medication, significantly reduced weight regain in patients who had undergone RYGB ( 1 ).

Specifically, anti-obesity medications stopped weight regain by around 10% relative to the patient’s nadir weight. They also reduced the risk of patients relapsing and rapidly regaining weight after bariatric surgery.

Those who regained the most weight were more likely to be “non-adherent”. Non-adherent patients had been prescribed anti-obesity drugs but missed follow up appointments. If they missed visits, it was deemed likely that their prescription had not been refilled, leaving them without the medication.

Istfan conculded that prescribing anti-obesity medications could help to prevent or treat weight regain post-operatively. This was confirmed by the finding that patients typically lost weight while taking the medication but also regained it when the drug was not prescribed ( 1 ).

There is currently limited guidance for doctors on initiating phentermine. Further studies are required to help professionals monitor its long-term effectiveness ( 1 ).

Phentermine for Weight Maintenance

Phentermine for Weight Maintenance

Phentermine is the most popular weight loss medication in the United States. It is a class IV controlled medication and is therefore available only when prescribed by a doctor. Phentermine works by reducing appetite and boosting energy levels. Combined, these two effects help to maintain a healthy weight following RYGB.

Phentermine is not suitable for every person wanting to maintain or lose weight. Since it has a different mode of action to other anti-obesity medications, a different class of anti-obesity medication may better suit some patients.

If you and your doctor decide that phentermine is the right choice, it is wise to be aware of any potential side-effects before you begin treatment. Some patients note a dry mouth or headache when taking the medication, but being aware of this beforehand can be beneficial.

Some people notice that long term use of phentermine can lead to a plateau in weight loss, or difficulty with weight maintenance. This is common as the body adjusts to the medication and develops a tolerance.

If phentermine tolerance occurs, it can help to take a break from the pills and come back to them. During this time, weight regain could occur. This is a crucial time for patients to follow the diet and exercise plan recommended by their bariatric team.

How to Achieve the Best Results With Phentermine

If the effects of phentermine are not as significant as expected, following simple steps can help you to regain control of your weight loss journey. As well as eating nourishing foods and completing aerobic and strength workouts, there are additional steps to maximize your energy potential.

  • Set goals with your bariatric team and regularly weigh-in. Having a plan for weight loss or maintenance can help you stay on track.
  • Prioritize sleep and relaxation. Aim for 7-9 hours of sleep each night to allow the body to repair. Consider adding in meditation or breathing exercises for excellent stress management.
  • Discuss with your medical team, whether additional vitamin supplementation, such as vitamin B12, could benefit you.
  • Phentermine is available in doses from 8mg – 37.5mg. Your doctor may agree that a higher dose is required after reviewing your serial weight measurements.


Bariatric surgery, including Roux-en-Y Gastric Bypass, is the most effective treatment for obesity. However, up to 25% of patients regain weight despite this major surgical procedure. Phentermine effectively mitigates weight regain, but the management guidelines are limited. Patients requiring pharmacological assistance with weight maintenance should discuss phentermine with their doctor.


  1. Istfan NW, Anderson WA, Hess DT, Yu L, Carmine B, Apovian CM. The Mitigating Effect of Phentermine and Topiramate on Weight Regain After Roux-en-Y Gastric Bypass Surgery. Obesity (Silver Spring). 2020 Jun;28(6):1023-1030. doi: 10.1002/oby.22786. PMID: 32441476; PMCID: PMC7250052.
  2. Clark A. (2019) Weight regain after bariatric surgery: Nutritional considerations.
  3. The Obesity Society (2020) Anti-obesity medications mitigate weight regain in RYGB surgery patients.
  4. Richardson WS, Plaisance AM, Periou L, Buquoi J, Tillery D. Long-term Management of Patients After Weight Loss Surgery. Ochsner J. 2009 Fall;9(3):154-9. PMID: 21603433; PMCID: PMC3096273.