GLP-1 weight-loss drugs can reduce weight and improve health, but can they also enhance athletic performance? Scientists say the evidence is unclear, and debate is growing over whether it can be considered a performance enhancer.
"The only opponent I couldn't overcome was weight," Serena Williams said on Oprah Winfrey's podcast last year. "I was training eight hours a day in the gym, and I couldn't lose a gram. In fact, I felt like I gained weight."
In the interview, she explained that she started using a weight-loss drug called GLP-1 after she had difficulty losing weight after the birth of her child.
In another interview with Vogue, she said the drug in question was tirzepatide (sold under the brand names Zepbound and Mounjaro).
Williams is paid as a brand ambassador for Raw, the company that supplies these drugs, and her husband, Alexis Ohanian, is an investor in the company.
Tirzepatide is one of several drugs available on the market that work to suppress appetite and potentially cause significant weight loss. These drugs work by mimicking the gut hormone GLP-1, and in the case of terzepatide, it also mimics GIB, both hormones that make us feel full.
In sport, any substance that gives a competitive advantage has long been subject to intense scrutiny by competitors, fans and anti-doping bodies.
With her return to professional tennis, Williams' decision to take the weight-loss drug GLP-1 raised questions about whether these drugs can improve athletic performance, or are they just a legitimate means to help an athlete lose weight and improve their health?
David Cowan, a global expert in doping detection and control at King's College London in the UK, explains that classifying any substance as a doping in sport requires meeting specific criteria.
These criteria include that the substance has the potential to enhance performance, poses a potential risk to the health of the athlete, or violates the 'spirit of the sport'.
While weight-loss drugs are not currently banned in professional sports, Cowan confirms that anti-doping agencies are paying close attention to this issue.
GLP-1 drugs are still included in the monitoring program of the World Anti-Doping Agency (WADA), and a spokesperson for the agency told the BBC, saying: “The monitoring program includes substances that are not on the list of prohibited substances, but we want to monitor them to detect any potential patterns of misuse in sport.”
Accordingly, the agency does not currently consider weight-loss drugs to be 'unfair' or performance-enhancing, according to Cowan.
One reason for this uncertainty is that the effect of GLP-1 drugs has not been studied in professional athletes, says Massimo Mapelli, a cardiologist and exercise physiologist at the Monzino Cardiology Center in Milan, Italy.
He adds: 'Things are evolving rapidly, and what we considered normal until now is changing.'
As research develops, there are several ways scientists and sports organizations will analyze how weight-loss drugs affect athletes.
One of the most direct methods occurs in sports where strict weight classes are applied.
In boxing and wrestling, for example, athletes compete within weight categories, and must reach the required weight limit to be allowed to compete. In these sports, weight-loss drugs can be considered a type of doping, according to Mapelli.
He adds: 'You are essentially taking a drug that may give you an advantage.'
For similar reasons, the use of diuretics, which are drugs that stimulate the body to urinate more and thus reduce weight in athletes, is prohibited. They can also be used to 'dispose of previously taken banned substances', according to the World Anti-Doping Agency (WADA).
Scientists are increasingly realizing that the effect of these drugs on the body goes beyond weight loss.
GLP-1 drugs have been found to reduce inflammation, improve metabolic health, and reduce the risk of heart attacks and strokes.
In other words, these drugs can improve a person's heart health independently of weight loss, but scientists don't know why.
Daniel Drucker, an endocrinologist at the University of Toronto in Canada, who is involved in pioneering research to develop GLP-1 drugs, says that the fact that these drugs have other benefits beyond weight loss is an important point to take into account.
For example, Williams has spoken publicly as a Raw ambassador about the health benefits she has seen.
Williams said her risk of cardiovascular disease decreased by 19 percent, the condition of her joints improved, and her cholesterol level decreased.
She added: “Honestly, I can now make movements that I could not do before.”
These benefits are likely to impact Williams' performance, Drucker says.
Drucker, who also advises a number of manufacturers of these drugs, adds: “I do not rule out the possibility that GLP-1 drugs may indirectly improve the efficiency of metabolism during exercise.”
For example, GLP-1 drugs improve blood flow to the muscles and help protect the health of the body's organs, and these benefits may be due to their role in reducing inflammation.
However, Abby Smith-Ryan, a professor of exercise physiology at the University of North Carolina at Chapel Hill, emphasizes that exercise itself is 'superior to medication' in reducing inflammation.
She adds: 'The strongest anti-inflammatory effects related to these drugs also appear in people who are overweight or obese. Therefore, if there is an effect on athletic performance, it is likely to appear first in athletes who are overweight or obese, rather than in thin athletes who are already in good shape.'
If small doses of these drugs are shown to significantly reduce inflammation in lean athletes, “they may have an indirect role in recovery, and perhaps in improving performance... but we are not there scientifically yet,” says Smith-Ryan.
Drucker explains that weight loss medications affect GLP-1 receptors in the brain, which in turn may lead to effects that protect other organs.
But he adds that these effects have not yet been studied in athletes in a way that allows answering the question of whether they can improve performance.
He continues, “Until then, I think we have to remain conservative and say that there is no data indicating that it enhances performance, but the absence of evidence is not evidence of the absence of an effect.”
Given the known adverse side effects, including loss of muscle mass, gastrointestinal problems and, less commonly, acute pancreatitis, rapid weight loss with GLP-1 drugs can be a risky move for an athlete.
Muscles are necessary to provide the body with the energy needed to move faster, improve endurance, and reduce the risk of injury, so loss of muscle mass may negatively affect performance.
However, if an athlete loses body fat but manages to maintain or increase his muscle mass, it may be beneficial, says Michael Joyner, an exercise physiologist and fellow of the American College of Sports Medicine.
He adds that continuous training can easily mitigate the effects of any loss of muscle mass.
'You would naturally expect any professional athlete using these drugs to control their weight to train harder,' he says.
He adds that 'the ability to exploit the smallest details' in professional sports is crucial.
Losing a few kilograms could be a “radical change that helps athletes move faster and improve their endurance.”
Research also indicates that the rate of muscle mass loss may vary depending on medications. One recent meta-analysis estimated that about 35 percent of the weight lost with semaglutide was muscle mass, including muscle, water, and other non-fat tissue, compared to about 25.4 percent with terzepatide.
The study stated that muscle mass “can be significantly preserved by incorporating resistance exercises, eating a sufficient amount of protein, and monitoring body composition.”
Regardless of whether GLP-1 agonists are considered doping or not, sports scientists say their use could complicate performance, as these drugs, in addition to their side effects, can make it more difficult to eat enough calories to support intense training.
“Nutrition is the foundation of performance, and its deficiency directly impairs training adaptation, recovery, and competitive performance,” says Smith-Ryan.
Many professional athletes already struggle to consume enough energy to meet the demands of their training.
She adds: “GLP-1 drugs work by influencing the brain to suppress appetite.” "Therefore, adding drug-induced decreased appetite to an already exhausting training load poses a real risk, as it may push athletes into further nutritional deficiencies, compromising energy availability, recovery and performance."
In a study published in July 2026, researchers examined 36 athletes who followed a low-calorie diet, while continuing to consume adequate amounts of protein and perform resistance exercises to maintain muscle mass, which is advice also given to those who take GLP-1.
Despite following these guidelines, athletes experienced a slowdown in metabolism, which could lead to fatigue. It is still unclear whether this same effect will occur with the use of GLP-1.
When athletes consume fewer calories than their energy needs, this may lead to a condition known as relative energy deficiency in sports. This, in turn, may affect bone health, mood, and athletic performance.
Williams returned to professional tennis after a four-year absence. She is scheduled to play her next match this week alongside her sister Venus Williams in the women's doubles competition at the Cincinnati Open.
She said she spoke publicly about her use of these medications in order to remove the stigma associated with them.
Although science has not yet found an accurate answer about how weight-loss drugs affect athletic performance, there are clear ethical concerns.
To begin with, it is rare for professional athletes to fall into the target group for these drugs, i.e. those with type 2 diabetes or weight-related diseases, with some exceptions.
Novo Nordisk, the maker of Wigovi and Ozambik, said in a statement to the BBC: “We do not support the use of our medicines for purposes outside their approved indications.”
As for Eli Lilly, the company that manufactures Zibound and Mongaro, it said: “Terzepatide is used, in addition to diet and exercise, for adults and adolescents (and children aged ten years and older) with type 2 diabetes, and for adults who suffer from obesity, overweight, or weight-related comorbidities, for weight management.”
“Medicines always have the potential for abuse, and can be harmful when misused,” Cowan warns.
In turn, Joyner says some athletes might find it unfair if others could more easily achieve a competitive advantage through medication, compared to lifestyle-change interventions.
“We cannot predict the benefits, risks, or performance effects that professional athletes may derive from GLP-1,” Mapelli says.
“Until more data becomes available, anti-doping agencies are operating without a clear vision,” says Drucker.
Critics say that Williams' role as an ambassador and strong advocate for the drug manufactured by the company "Roe" sets a disturbing precedent.
Williams is shown on the company's website smiling and injecting herself with a weight-loss drug.
“The aim of putting Williams in the ad is to normalize the use of weight-loss drugs as lifestyle products for people who are not typical patients,” says sports journalist Brian Armin Graham, writing in The Guardian.
Body image campaigner Alex Light writes in The Independent that its use of GBL-1 represents a “worrying cultural turning point” that shows women “remain trapped in society’s obsession with thinness.”
Celebrated as one of the greatest female tennis players of all time, Williams is a role model for girls and athletes around the world.
Mapelli worries that the use of these drugs by a famous athlete might suggest to teens that taking them is an acceptable option even for those of a healthy weight.
Until the science improves, Joyner believes we will see an increase in their use, both inside and outside of professional sports. If we see this increase, the debate over the concept of integrity in sport is likely to continue.
