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The injectable drug Mounjaro performed well in a weight loss clinical trial. Design by MNT; Photography by The Washington Post/Getty Images
  • Study participants lost a total of 26% of body weight when drug therapy was combined with lifestyle intervention.
  • Mounjaro, or tripeptide, is approved for treating type 2 diabetes but can also be prescribed off-label for weight loss.
  • The drug targets parts of the brain involved in hunger and satiety.

A group of people with obesity who undertook an intensive lifestyle intervention followed by treatment with the appetite inhibiting drug tirzepatide lost 26% of their body weight in the course of the 84-week program, according to a study conducted by drugmaker Eli Lilly.

The findings were presented at the 2023 annual meeting this week of The Obesity Society and were published in the journal Nature Medicine.

“[P]articipants — who had already lost 6.9 percent of their baseline body weight with [12 weeks of] traditional diet and activity counseling — lost an additional 18.4 percent of body weight when administered tirzepatide, compared with a gain of 2.5 percent in participants assigned to [a] placebo” group, said Dr. Thomas Wadden, the study lead author and a professor of psychology in psychiatry at the University of Pennsylvania’s Perelman School of Medicine, in a press statement. “The additional weight loss produced further improvements, compared with placebo, in multiple measures of health, including waist circumference, blood pressure, cholesterol and triglycerides, blood sugar and physical functioning.”

The initial weight loss was recorded over a 72-week period. There was another small decrease in weight loss during an additional 12-week period for a total of 26% over 84 weeks.

Tirzepatide, sold under the brand name Mounjaro, is approved by the Food and Drug Administration (FDA) to treat type 2 diabetes but also may be effective when prescribed as a treatment for obesity, past studies have shown.

“Obesity medicine physicians have seen a substantial increase in the off-label use of tirzepatide for the treatment of obesity,” Dr. Aleem Kanji, an endocrinologist with Ethos Endocrinology in Houston, Texas, told Medical News Today. “Approval of tirzepatide for the treatment of obesity by the FDA would ideally increase access to those who would benefit from treatment.”

“Tirzepatide is actually not a diet drug,” Dr. Kristen Williams, the medical director of weight-loss program MyJourneyMD, told Medical News Today.

However, she said, “Compared to previous FDA approved drugs, the results seen thus far are very exciting for the treatment of obesity. Success has been generally described as 10 percent body weight, so to see numbers above 25 percent are truly remarkable. We may finally be addressing the metabolic arm of obesity.”

The new findings about the effectiveness of the drug combined with a behavioral intervention included participants in the SURMOUNT-3 study, a multi-center, randomized, double-blind, parallel, placebo-controlled trial involving people with obesity as well as individuals who were overweight and had weight-related co-morbidities other than diabetes.

“These findings suggest that tirzepatide may be a potential game-changer for the treatment of obesity and type 2 diabetes, as it offers superior weight loss and glycemic benefits compared to existing therapies,” said Dr. Alex Foxman, the medical director of the Beverly Hills Institute, a weight-loss program. “However, more studies are needed to confirm the long-term safety and efficacy of tirzepatide, as well as its effects on cardiovascular outcomes and mortality. Additionally, it is important to note that tirzepatide is not a magic bullet for obesity and diabetes, and that lifestyle interventions, such as diet and exercise, are still essential for optimal health outcomes.”

Catherine Rall, a registered dietitian, told Medical News Today that it’s important to remember that researchers “didn’t just give people this drug and stick them on a scale occasionally. The study also included regular counseling sessions with a registered dietitian in order to help participants maintain a balanced diet with a calorie deficit and get regular exercise.”

“While participants in the study who received a placebo lost less weight than those who received tirzepatide, most of them still did lose weight,” she noted. “This is not to say that tirzepatide isn’t effective, but it’s an important reminder that diet and exercise — and professional help in managing diet and exercise — are essential ingredients in anyone’s weight-loss journey.”

The side effects associated with tirzepatide also need to be taken into account, said Dan Gallagher, a registered dietitian with dietary supplement maker Aegle Nutrition.

“Nausea, vomiting, face swelling, stomach pain, diarrhea, and constipation are common side effects of this drug,” Gallagher told Medical News Today. “The other issue is what happens when you stop taking the drug. Does the weight come back?”

Some of the reported weight loss reported also may be muscle mass rather than the desired body fat, he added.

Administered once a week, tirzepatide activates receptors for the hormones GIP and GLP-1, located in parts of the brain involved in appetite regulation. The drug has been shown to decrease food intake by increasing satiety and decreasing hunger and reward-based eating.

“The net effect is that patients feel less hungry and feel full for a longer period of time,” said Dr. Mir Ali, a bariatric surgeon and medical director of the MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in California.

“If a patient meets the criteria for surgical intervention, that is still the most effective treatment,” Ali told Medical News Today. ”If they are diabetic and don’t meet criteria for surgery, this medication can be very helpful.”