In a major finding for people considering weight loss treatment, a new study comparing the two injectable prescription medications has show one of two noted brands to be more effective for weight loss than the other.
While the risk of side effects from the drugs is comparable, a research team in the journal JAMA Internal Medicine found that tirzepatide is more effective for weight loss than semaglutide.
Tirzepatide is manufactured by the US pharmaceutical company Eli Lilly under the brand name Mounjaro for diabetes and Zepbound - the same drug with the same strength - for weight management, while Denmark-headquartered Novo Nordisk makes semaglutide as Ozempic for diabetes and Wegovy - a higher-dose version - for weight management.
Both have been used for quite some time now to treat type 2 diabetes and, since 2023, for weight loss in adults who are overweight or obese - along with a reduced-calorie diet and increased physical activity.
Researchers from Truveta, a health data analytics company based in the US city of Bellevue, Washington, analysed electronic health records for more than 18,000 adults who were overweight or obese and treated in the US with either tirzepatide or semaglutide. Their mean age was 52 years, and mean weight 110 kilograms at the start of their treatment.
The study investigated whether the patients had achieved a weight loss of at least 5%, 10% or 15% after three, six and 12 months. While both medications proved effective, significantly more patients who used tirzepatide lost weight, and their weight loss was on average greater, the researchers found.
The results bolster evidence from earlier studies, according to the researchers.
Specifically, the new study found that nearly 82% of the patients who took tirzepatide lost at least 5% of their starting body weight after one year, compared with about 67% of those who took semaglutide. A reduction of 15% or more after one year of use was reached by about 42% of the tirzepatide patients, as against 18% of those who took semaglutide.
Rates of gastrointestinal adverse events - such as nausea, diarrhoea or constipation - were found to be similar between the two groups.
Consistent with clinical trials, the researchers found larger weight reductions among patients without type 2 diabetes, compared with those who had it. "The underlying reasons are unclear," they write, and suggest the former may have greater motivation to lose weight - and act accordingly.
Speaking to the study's limitations, the researchers note it included medications labelled for type 2 diabetes only and that "future studies are needed to compare versions labelled for weight loss." Health records of about half of the patients showed no diabetes diagnosis, so they were probably using the drugs "off-label" for weight management.
More work is also needed, the researchers say, to compare the effect of tirzepatide and semaglutide on reducing obesity complications such as major adverse cardiovascular events.
More the half the patients in the study discontinued treatment, they point out, and say additional research on this is needed. The discontinuation rate was similar for both tirzepatide and semaglutide patients. As possible reasons, the researchers mention unwanted side effects, shortages of the medications or high costs.
The high rate is particularly concerning as studies have shown that a "yo-yo effect" - a cyclical regain and loss of weight - typically follows a break-off of weight-loss measures. Tirzepatide and semaglutide must be taken continually to have a lasting effect, their long-term effects yet unclear.
Both drugs are glucagon-like peptide-1 (GLP-1) receptor agonists and mimic the effects of the GLP-1 hormone, which stimulates the body's production of insulin when blood sugar is high, slowing down passage of food through the stomach. This helps reduce appetite and food intake, hunger attacks and craving for fatty foods.
The drugs have caused a sensation on social media in recent months, in part because a number of US celebrities and influencers say they've lost weight by taking them.
Wegovy (semaglutide) was approved in the US in 2021 as a weight-loss treatment for obese (body mass index of 30 or greater) or badly overweight (BMI of 27+) adults with at least one weight-related ailment such as diabetes or high blood pressure. In early 2022, the EU also approved it for weight loss and control.
Mounjaro (tirzepatide) was approved for this purpose in the US and UK - sold under the brand name Zepbound for the expanded label - and EU late last year.
Both semaglutide and tirzepatide are meant to be injected once weekly from pre-filled pens under the skin of the thigh, abdomen or upper arm.
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