Serena Williams has revealed she wishes she had taken a GLP-1 weight-loss drug while still on tour, saying it would have made a 'big difference' to her career, as debate grows in elite sport over whether these medicines may affect muscle mass as well as body fat.

Williams, 45, a 23-time Grand Slam singles champion, made the remarks on The Oprah Podcast in January. 'I just wish I had done this while I was still playing,' she said, adding that it would have made a 'big difference' to her career.

Williams, who stepped away from professional tennis in 2022 before returning to the WTA Tour in 2026, disclosed in August 2025 that she was using Zepbound, a tirzepatide drug, through Ro.

She has said she has lost 31lb (14kg), which she said has eased pressure on her joints. She now receives her GLP-1 treatment through the telehealth company Ro, with which she has a partnership. Her husband, Alexis Ohanian, is an investor in Ro.

Muscle Loss at the Centre of GLP-1 Concerns

GLP-1 agonists such as semaglutide and tirzepatide mimic gut hormones to suppress appetite and drive substantial weight loss. Developed for type 2 diabetes and obesity, they are increasingly used in fitness and sport, where a few kilograms can mean prize money, contracts or Olympic qualification.

Studies have found that a substantial proportion of weight lost during treatment with GLP-1 drugs can come from lean mass, which includes muscle. For athletes, that could have implications for performance.

Under-fuelling caused by reduced appetite could also contribute to low energy availability and relative energy deficiency in sport (RED-S), a syndrome that can affect bone health, immunity and performance.

Regeneron, a US drugmaker, is attempting to address the problem. Data presented at the European Association for the Study of Diabetes conference in Milan showed that its experimental drug trevogrumab, which blocks the muscle-regulating protein myostatin, roughly halved lean-muscle loss when combined with Novo Nordisk's semaglutide.

In a trial of nearly 1,000 adults with obesity, scans showed a 6.7 per cent loss of lean mass over 26 weeks on semaglutide alone. Adding a 75mg dose of trevogrumab cut that to 3.3 per cent.

Boaz Hirshberg, a Regeneron executive, said the company had 'confirmed' that people on GLP-1s lose about 30 per cent of their weight as lean mass. Investors have forecast that muscle‑preserving add‑ons could become a multibillion‑dollar market.

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Wada Scrutiny, Scientific Doubts and Ethical Questions

The World Anti-Doping Agency (Wada) has not banned GLP-1 drugs. Semaglutide was added to its monitoring programme in 2024, and Wada's 2026 programme monitors markers of both semaglutide and tirzepatide. To be prohibited, a substance must meet at least two of three criteria, enhancing performance, posing a health risk or violating the 'spirit of sport'.

Scientists urge caution. Daniel Drucker, a University of Toronto endocrinologist and pioneer of GLP-1 research, said there was 'insufficient data' to call the drugs performance‑enhancing or to dismiss the idea. Oliver Catlin, an anti‑doping campaigner, said Williams was 'literally experiencing the performance‑enhancing benefits' of the therapy.

A Wada‑funded randomised trial in Copenhagen is following about 40 high‑level endurance runners over four and a half months, monitoring 5km times, muscle mass and mental state.

A 2026 Loughborough University-led survey found that 12 per cent of sports medicine physicians were aware of athletes using GLP-1 drugs without a medical need, with reported use spanning sports including cycling, swimming, equestrian and bodybuilding.

Williams has argued that speaking openly about GLP-1 use helps remove stigma. Critics say her partnership with Ro presents powerful drugs as lifestyle products to people for whom they are not intended.

Drucker said there was no firm evidence that GLP-1s enhance performance in elite competitors. Whether that changes will depend on research now under way.

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