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Could a drug designed for weight loss help athletes perform better?
18th August, 2026 01:05 AM
GLP-1 weight-loss drugs can reduce weight and improve health, but could they also boost athletic performance? Scientists say the evidence is unclear, but there is a growing debate around whether they could be considered performance-enhancing.
“The one opponent I could not defeat is the weight,” Serena Williams said on Oprah Winfrey’s podcast last year. “I literally was working eight hours a day in the gym training, and I never could lose an ounce. In fact, I feel like I may have even gained weight,” she said.
In the interview, she explained she began taking a GLP-1 weight loss drug after struggling to lose weight after her child was born. In an interview with Vogue, she said the drug in question was tirzepatide (which is sold under brand names Zepbound and Mounjaro). Williams is paid to be a brand ambassador for Ro, a company that administers the drugs, and her husband, Alexis Ohanian, is an investor in the company.
It’s one of several drugs on the market that suppress appetite and can lead to substantial weight loss. The drugs work by mimicking the gut hormone GLP-1, and in the case of tirzepatide, also GIP – both are hormones that make us feel full.
In sports, it’s long been the case that any substance that gives you an edge is heavily scrutinised by opponents, spectators and anti-doping agencies. As she returns to professional tennis, Williams’s decision to take a GLP-1 weight-loss drug has led to questions about whether they could improve athletic performance or are a legitimate way for an athlete to lose weight and improve their health. But should they be considered performance-enhancing?
What is considered doping?
To be classified as doping in sports, a substance needs to meet specific criteria, explains David Cowan, a world expert on drug detection and anti-doping at King’s College London, UK. They need to have the potential to enhance performance, have the potential to risk the health of the athlete, or they violate the “spirit of sport”.
While weight-loss drugs are not currently banned in professional sports, Cowan says doping agencies are paying close attention. GLP-1s remain on the World Anti-Doping Agency’s (Wada) list of drugs to monitor. “The Monitoring Program includes substances which are not on the prohibited list, but that Wada wishes to monitor to detect potential patterns of misuse in sport,” a spokesperson told the BBC.
With that in mind, weight-loss drugs are not currently considered “unfair” or performance-enhancing by Wada, says Cowan.
One reason for the uncertainty is that GLP-1 drugs have not been studied in elite athletes, says Massimo Mapelli, a cardiologist and exercise physiologist at the Centro Cardiologico Monzino in Milan, Italy. “Things are rapidly evolving and what we have considered normal until now is changing,” he says.
Benefits beyond weight loss
Scientists have increasingly realised that the drugs affect the body beyond weight loss. GLP-1 drugs have been found to reduce inflammation, improve metabolic health and reduce the risk of heart attacks and strokes, with weight loss only accounting for one-third of the reduced cardiovascular risk, according to a recent Lancet study.
In other words, they can improve an individual’s heart health independent of weight loss, but scientists don’t know why.
Daniel Drucker, an endocrinologist at the University of Toronto, Canada, who was involved in pioneering research in the development of GLP-1 drugs, says that the fact that GLP-1s have other benefits beyond weight loss is an important point to consider.
Williams, for instance, has spoken publicly in her role as ambassador for Ro about the health benefits she has experienced. Williams said that her cardiovascular risk had decreased by 19%, her joints had improved, and her cholesterol had improved. “I can pull moves I could, quite frankly, never do before,” she said.
Adverse side-effects
Given the now well-established adverse side-effects, including muscle loss, gastrointestinal issues and, more rarely, severe inflammation of the pancreas, losing weight rapidly on GLP-1s could be a risky move for an athlete.
Muscles are required to power the body to move faster, improve endurance and reduce the risk of injury, so muscle loss could clearly be detrimental to performance.
However, if an athlete loses body fat but is able to maintain or gain muscle, this could benefit them, says Michael Joyner, a specialist in exercise physiology and a fellow of the American College of Sports Medicine. Ongoing training could easily mitigate the effects of any muscle loss, he says.
“You would certainly expect any elite athlete who was using these drugs to control their body weight would also be training very hard,” he says. And in elite sport “being able to leverage tiny margins” is extremely important, he adds. Losing a few kilos can be “transformative to help people move around faster and improve their endurance”.
Research also suggests that the proportion of lean mass lost may vary between drugs. One recent meta-analysis estimated that around 35% of the weight lost with semaglutide was lean mass – which includes muscle, water and other non-fat tissues, compared with about 25.4% with tirzepatide. Muscle mass, the study wrote, “can be significantly preserved by integrating resistance training, adequate protein intake, and body composition monitoring”.
Under-fuelling
Whether or not GLP-1s will be considered doping, sports scientists say their use could actually impede performance, as in addition to the side effects, the drugs can make it harder to consume enough calories to support intense training.
“Fuelling is everything for performance; under-fuelling directly undermines training adaptation, recovery, and competitive output,” says Smith-Ryan.
Many elite athletes already struggle to consume enough energy to fuel their training demands. “GLP-1 drugs work by acting on the brain to suppress appetite. So layering a drug-induced drop in appetite on top of an already-demanding training load is a real risk: it can push athletes further into under-fuelling, which threatens energy availability, recovery, and performance,” she says.
Ethical concerns
Williams has now returned to professional tennis after four years away. Her next game is this week alongside her sister, Venus Williams, at the Cincinnati Open in women’s doubles. She spoke out, she said, to lift the stigma around using the drugs.
While the science hasn’t caught up to answer exactly how a weight-loss drug affects athletic performance, there are clear ethical concerns.
For starters, it’s rare for professional athletes to fall into the category the drugs are intended for – those with type 2 diabetes or a weight-related condition, though there are some exceptions.
