GLP-1 Weight Loss Drug Controversy in Professional Sports: Performance Enhancement or Health Management?

"The only thing I can't beat is my weight," Serena Williams told Oprah Winfrey's podcast last year. "I was literally training eight hours a day in the gym and I couldn't lose any weight. In fact, I felt like I might have even gained weight," she said.

In the interview, she explained that she had difficulty losing weight after the birth of her child, so she started taking the GLP-1 weight loss drug. She told Vogue in an interview that the drug she was taking was tirzepatide (trade names include Zepbound and Mounjaro). Williams was hired as a brand ambassador for Ro, a company that provides such drugs, and in which her husband, Alexis Ohanian, is an investor.

This is one of several medications on the market that suppresses appetite and can lead to significant weight loss. These drugs work by mimicking the gut hormone GLP-1, and tilpotide also mimics GIP—both hormones that make people feel full.

In the world of sports, any substance that offers an advantage has long been closely scrutinized by opponents, spectators and anti-doping agencies. As Williams returns to professional tennis, her decision to use GLP-1 weight-loss drugs has raised questions about whether such drugs can improve performance and whether they are a legitimate way for athletes to lose weight and improve their health.

But the question is, should they be considered performance-enhancing drugs?

What is doping?

David Cowan, a world-leading expert on drug testing and anti-doping at King's College London, explains that to be classified as a banned substance in sport, a substance needs to meet certain criteria. The substances must have performance-enhancing potential, be potentially harmful to the athlete's health, or be contrary to the "spirit of sportsmanship."

While weight-loss drugs are not currently banned in professional sports, Cohen said anti-doping agencies are paying close attention to developments. GLP-1 drugs are still included in the list of drugs monitored by the World Anti-Doping Agency (WADA). "The substances covered by the monitoring scheme are not on the banned list, but WADA wishes to monitor them to identify possible patterns of abuse in sport," a spokesman told the BBC.

With this in mind, Cohen noted that WADA does not currently consider weight-loss drugs to be "unfair" or have performance-enhancing effects.

One reason for this uncertainty is that GLP-1 drugs have not been studied in top athletes. "The situation is evolving rapidly, and what we have considered normal until now is changing," said Massimo Mapelli, a cardiologist and exercise physiologist at Centro Cardiologico Monzino in Milan, Italy.

As research continues to develop, scientists and sports organizations will analyze how weight loss drugs may affect athletes from multiple angles. One of the most immediate effects may be in sports with strict weight classes.

For example, in sports such as boxing and wrestling, athletes compete in weight classes and must meet a prescribed weight limit before they can compete. Mapelli said weight-loss drugs could be considered banned substances in these sports. "You're essentially using a drug that may give you an advantage," he said.

The use of diuretics has been banned for similar reasons. Diuretics cause the body to produce more urine, allowing athletes to lose weight. WADA points out that such drugs can also be used to "flush previously taken prohibited substances."

Benefits beyond weight loss

Scientists are increasingly aware that such drugs have an impact on the human body beyond weight loss. According to a recent study published in The Lancet, GLP-1 drugs can reduce inflammation, improve metabolic health, and reduce the risk of heart attacks and strokes; weight loss alone explains only about one-third of the reduction in cardiovascular risk.

In other words, these drugs can improve an individual's heart health without relying on weight loss, but scientists still don't know why.

Daniel Drucker, an endocrinologist at the University of Toronto in Canada, was involved in pioneering research on the development of GLP-1 drugs. GLP-1 drugs may have benefits beyond weight loss that are worth considering, he said.

Take Williams, for example, who has spoken publicly about the health benefits she has experienced as a brand ambassador for Ro. Williams said her risk of cardiovascular disease dropped by 19 percent, her joint condition improved and her cholesterol levels improved. "I was even able to do some moves that, frankly, I've never done before," she said.

Drucker said these effects could potentially affect Williams' athletic performance.

"I wouldn't rule out the possibility that GLP-1 drugs may, in some indirect way, make the body's metabolic environment more conducive to exercise," said Drucker, who also serves as a consultant for several companies that produce related drugs. For example, GLP-1 drugs improve blood circulation to muscles and protect the health of body organs. These benefits may be related to its effect in reducing inflammation.

However, Abbie Smith-Ryan, a professor of exercise physiology at the University of North Carolina at Chapel Hill, said exercise itself is "better than drugs" when it comes to reducing inflammation.

She said: "The most significant anti-inflammatory effects of these drugs are also mainly seen in people who are overweight or obese. Therefore, if they do have an effect on improving sports performance, it is likely to be first seen in overweight or obese athletes, rather than lean athletes who are already in good physical condition."

Smith-Ryan said that if low doses of these drugs turn out to significantly reduce inflammation levels in lean athletes, "then they may indirectly aid recovery and, in turn, impact performance."

"But from a scientific perspective, we're not there yet," she said.

Drucker explains that the weight-loss drug acts on GLP-1 receptors in the brain, thereby protecting other organs.

However, Drucker added that these effects have not yet been studied in athletes, so "it is currently impossible to answer whether they improve performance."

"Until then, I think we can only maintain a neutral stance and say that there is no data to show that they have a performance-enhancing effect. But the lack of evidence does not mean that there is no such effect."

adverse side effects

Rapid weight loss with GLP-1 drugs can be a risky proposition for athletes, given the now-well-publicized adverse side effects, including muscle loss, gastrointestinal problems, and, more rarely, serious pancreatic inflammation.

Muscle is necessary to propel the body faster, increase endurance, and reduce the risk of injury, so muscle loss can obviously impair athletic performance.

However, Michael Joyner, a fellow of the American College of Sports Medicine and an expert in exercise physiology, said that if athletes can lose body fat while maintaining or even increasing muscle mass, this may actually bring benefits. He points out that the effects of any muscle loss can be easily mitigated by consistent training.

"You would certainly expect that any top athlete using these drugs to control their weight would also be training at a very high intensity," he said. He added that in elite sports, "being able to take advantage of small gaps" is extremely important. Losing a few kilograms could have "transformative effects, helping people move faster and improve endurance".

Research also shows that the proportion of lean body mass loss caused by different drugs may be different. A recent meta-analysis estimated that about 35% of the weight lost with semaglutide (also known as "semaglutide") was a loss of lean body mass, which includes muscle, water and other non-fat tissue; compared with about 25.4% with tirzepatide. By combining resistance training with adequate protein intake and monitoring body composition, "muscle mass can be preserved to a large extent," the study states.

Calorie deficiency

As for whether GLP-1 drugs will eventually be considered banned, sports scientists point out that their use may actually hinder athletic performance because in addition to side effects, these drugs may also make it more difficult for athletes to consume enough calories to support high-intensity training.

"Energy is critical to athletic performance; inadequate caloric intake directly impairs training adaptation, physical recovery, and competitive performance," Smith-Ryan said.

Many top athletes already struggle to consume enough energy to meet training demands.

"GLP-1 drugs suppress appetite by acting on the brain. Therefore, adding drug-induced loss of appetite on top of an already heavy training load does pose a risk: this may further push athletes into a state of insufficient energy intake, thereby threatening energy supply, recovery and performance," she said.

In a study published in July 2026, researchers looked at 36 athletes on a calorie-restricted diet. These athletes still eat plenty of protein and perform resistance training to preserve muscle mass, which is what is often recommended for people taking GLP-1 drugs. Despite following these guidelines, these athletes still experience a slowed metabolism, which can lead to fatigue. It is still unclear whether the same effect will occur when taking GLP-1 drugs.

When athletes do not consume enough calories to meet their energy needs, it can also lead to a condition called Relative Energy Deficiency in Sport (Red-S). This can affect bone health, mood, and athletic performance.

Professional ethics concerns

Williams is now back on the court after four years away from professional tennis. Her next match will be this week at the Cincinnati Open in the women's doubles event, where she will partner with sister Venus Williams.

She said she spoke publicly about the matter in the hope of removing the stigma surrounding the use of such drugs.

Although scientific research has not yet determined exactly how weight loss drugs affect athletic performance, the related ethical issues are quite obvious.

First, professional athletes rarely fall into the group for which these drugs are intended, namely people with type 2 diabetes or weight-related conditions, although there are some exceptions.

Novo Nordisk, which produces Wegovy and Ozempic, issued a statement to the BBC saying: "We do not support the use of our drugs for purposes other than their approved indications."

Eli Lilly, which produces Zepbound and Mounjaro, said: "Tirzepatide is suitable for adults, adolescents and children aged 10 or above with type 2 diabetes, and must be combined with diet control and exercise; it is also suitable for adults with obesity, or who are overweight and have weight-related comorbidities, for weight management."

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

These medicines are prescription drugs but are also easily available through private healthcare providers online. This easy access raises concerns among doctors, especially given the large number of unregulated GLP-1 drugs on the market.

So if some athletes are able to gain an advantage more easily through drugs while others rely on lifestyle adjustments, other athletes may view the situation as unfair, Joiner said.

Bryan Armen Graham, a sports reporter for The Guardian, wrote that "the purpose of using Williams in the ad is to normalize weight-loss drugs as a lifestyle product for those who are not typical sufferers." Alex Light, a body image advocate, wrote in The Independent that her use of GLP-1 drugs marked a "disturbing cultural turning point" and showed that women "are still trapped in society's obsession with thin bodies."

Williams is hailed as one of the greatest tennis players of all time and a role model for girls and athletes around the world. Mapelli worries that if a high-profile athlete uses such drugs, it could send a message to teenagers that drugs are an acceptable option even at a healthy weight.

Joyner believes that until scientific research catches up with reality, we will see a continued increase in the use of these drugs both inside and outside professional sports. And if this growth occurs, the debate over fairness in sport is likely to continue.