Could a drug designed for weight loss help athletes perform better?

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?

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 in order 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 in Italy. "Things are rapidly evolving and what we have considered normal until now is changing," he says.

As research develops, there are several ways scientists and sports organisations will be analysing how weight-loss drugs could affect athletes. One of the most direct ways could be in sports where there are strict weight categories.

In boxing and wrestling, for instance, athletes compete in weight categories and must meet the required weight limit to compete. In those sports, weight-loss drugs could be considered doping, says Mapelli. "You're essentially taking a drug that could give you an advantage," he says.

For similar reasons, the use of diuretics – drugs that cause the body to pass more urine and therefore make athletes lighter – are banned. They can also be used "to flush previously taken prohibited substances," Wada state.

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.

It's possible these effects could affect Williams' performance, says Drucker.

"I wouldn't rule out the possibility that GLP-1s are somehow indirectly making the metabolic environment more efficient for exercise," says Drucker, who also consults for several companies which make the drugs. For instance, GLP-1s improve blood flow to muscles and protect the health of our organs. These benefits could be due to their role in decreasing inflammation.

Exercise itself, however, "outperforms the drug" at reducing inflammation, says Abbie Smith-Ryan, professor of exercise physiology at the University of North Carolina at Chapel Hill.

"The strongest inflammation effects from these drugs also show up in people with overweight or obesity, so if there's a performance angle, it'd likely surface in overweight or obese athletes first, not lean, already-fit ones," she says.

If small doses of these drugs do turn out to meaningfully reduce inflammation in lean athletes, "there could indirectly be a role in recovery, and potentially performance", says Smith-Ryan. "We're just not there yet scientifically."

Weight-loss drugs, Drucker explains, act on GLP-1 receptors in the brain, which can then go on to protect other organs. However, these effects have not yet been studied in athletes "to be able to answer whether they can improve performance", adds Drucker.

"Until then we just have to, I think, sit on the fence and say there's no data to suggest they're performance-enhancing, but absence of evidence is not evidence of absence."

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".

Whether or not GLP-1s are 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.

In a study published in July 2026, researchers looked at 36 athletes on a calorie-restricted diet who still consumed plenty of protein and took part in resistance training in order to preserve muscle – which is also advice given to those taking GLP-1s. Despite following these guidelines, the athletes experienced slower metabolisms, which could result in fatigue. Whether this same effect would occur on GLP-1s is unclear.

When athletes consume too few calories to meet their energy needs it can also result in a condition called Red-S (relative energy deficiency in sport). This can affect bone health, mood and athletic performance.

Williams has now returned to professional tennis after four years away. Her next game takes place this week alongside her sister Venus Williams at the Cincinnati Open in the women's doubles. She spoke out, she said, to lift the stigma around using the drugs.

While the science simply 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.

Novo Nordisk, the manufacturer behind the drugs Wegovy and Ozempic, said in a statement to the BBC: "We do not support the use of our medicines for purposes that fall outside their approved indications." Eli Lilly, the manufactory behind Zepbound and Mounjaro stated that: "Tirzepatide is indicated, alongside diet and exercise, for adults, adolescents and children aged 10 years and above with type 2 diabetes, and for adults with obesity or overweight and a weight-related comorbidity for weight management."

Neither company commented on the use of the drugs by professional athletes.

They are prescription-only medications, but they are easily accessed by private healthcare providers online. This easy access has concerned physicians, especially given the surge in unregulated GLP-1 drugs available on the market. "Drugs always have a misuse potential and may be harmful when misused," cautions Cowan.

Joyner therefore says that it might be considered unfair by other athletes, if some more easily reach a competetive advantage through drugs, compared to lifestyle interventions.

We can't assume what benefits, risks or effects on performance elite athletes might gain from GLP-1s, says Mapelli. But until we have more data, doping agencies are essentially "flying blind" says Drucker.

Critics argue that Williams' role as ambassador and vocal advocate for Ro sets a worrying precedent.

More like this:

• What you need to know before taking weight-loss drugs

• Is it possible to lose weight on an 'Ozempic' diet?

• What happens when you stop taking weight-loss drugs?

On the company's website she is pictured smiling, injecting herself with a weight-loss drug. Writing in the Guardian, sports journalist Bryan Armen Graham says that "the point of putting Williams in the ad is to normalise weight-loss drugs as lifestyle products for people who aren't typical patients." Body image campaigner Alex Light writes in the Independent that her use of GLP-1s marks a "disturbing cultural turning point" showing that women "remain trapped by society's obsession with thinness".

Williams – who is celebrated as one of the greatest tennis players of all time, is a role model for girls and athletes worldwide. Mapelli is concerned that if an athlete in the public eye uses these drugs, it could send a signal to teenagers that medication is a valid option even for those who are a healthy weight.

Until the science catches up, Joyner believes we are going to see an increase in their use, both in and outside professional sports. And if we see this increase, the debate around what is fair in sports is likely to continue.

* Melissa Hogenboom is a senior health correspondent at the BBC and author of Breadwinners and The Motherhood Complex. She is melissa_hogenboom on Instagram. 

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