Serena Williams has won 23 Grand Slam singles titles, has twice held all four major titles at once and inspired a generation of tennis players. It’s a run that, to hear her tell it, represents a tale of unfulfilled potential.
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In January, more than three years after she retired from tennis, and months before she unretired, Williams, who just turned 45, told Oprah Winfrey that her use of the weight-loss drug Zepbound had changed her life to a degree that she looked back on her tennis stardom with regret.
“I’m like, ‘Oh my goodness, I just wish I had done this while I was still playing,’” Williams gushed on “The Oprah Podcast.” “It would have made such a big difference for me in my career.”
When Williams announced last year that she had joined some 15 percent of Americans in taking a GLP-1 agonist, she became the most prominent athlete to openly use the diabetes-turned-weight-loss drug. Williams has said she dropped 34 pounds in her first year taking it, relieved stress on her joints and found herself able to move more freely than in years.
Her effusive praise opened a window into a debate that is quietly simmering among athletes and officials across professional and amateur sports: Are GLP-1s performance-enhancing drugs?
For now, the World Anti-Doping Association says no. As long as a doctor prescribes the GLP-1, athletes are allowed to take it. (As with all drugs, it is banned if taken illegally.)
WADA did add GLP-1s to its monitoring list in 2024, tracking its usage across sports. For a drug to move from there to the prohibited list, it must meet two of three criteria: It can enhance performance, poses a health risk and violates the spirit of sports. At its annual review this month, WADA declined to ban GLP-1s.
But the agency is looking into it, WADA has said, including funding one study, in Copenhagen, where a randomized controlled trial of some 40 high-level endurance runners is spending 4½ months tracking metrics including their 5K time, their muscle mass and their mental state. They hope to have results ready to publish in about a year, said lead researcher Morten Hostrup.
Scientists across a variety of disciplines are doing additional research, trying to sort out whether and how these drugs might benefit athletes — and just how many athletes are already using them.
For one prominent anti-doping activist, Williams’s words could have been the end of the conversation.
“That’s an athlete who’s literally experiencing the performance-enhancing benefits of it as we speak,” said Oliver Catlin, the president of Banned Substances Control Group. “I don’t know what better to use than the actual words of an athlete and the impacts on their performance from their own perspective.”
Scientists, though, say it’s more complicated than that.
“There’s just insufficient data to come down on either side of this,” said Daniel Drucker, a University of Toronto endocrinologist who helped discover GLP-1s. “It’s a great question.”
It’s also an increasingly pressing one: A study by scientists at Britain’s Loughborough University found that 12 percent of the sports medicine physicians they surveyed knew of elite athletes who were using GLP-1s without medical need, including jockeys and other equestrians, badminton players, cyclists, swimmers, volleyball players, and bodybuilders.
Daniel Read, a doping researcher who led the study, believes those figures are low. Retired sports figures including Charles Barkley, Billie Jean King and Shaquille O’Neal have all said they have used the drugs, but there is little upside for active athletes to acknowledge their use.
Experts can imagine the sort of athlete who might use one — an NFL lineman returning from a knee injury, a boxer trying to drop a weight class — but they do not expect any of them to reveal their medical information unless, as is the case with Williams, there is a financial incentive. (Williams’s husband, the entrepreneur Alexis Ohanian, is an investor in Ro, the telehealth company through which Williams obtains her Zepbound.)
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If they have a prescription, they face the stigma of being unable to lose weight strictly through diet and exercise. If they don’t have a prescription, they are breaking the rules. And either way, at any time, WADA could ban the drugs entirely, and then they would have to answer questions about that.
Banning them might not be so easy, though: It is difficult to prohibit a drug for which you cannot reliably test.
Synthetic GLP-1s mimic naturally occurring glucagon-like peptide-1, a hormone that helps lower blood sugar and trigger satiety. They exit the body in hours to weeks, depending on the drug. An athlete who chooses a version with a short half-life and times the dose correctly could take it in the morning, race in the afternoon and pass a drug test just off the finish line.
Meanwhile, scientists are not even sure the benefits outweigh the drawbacks for elite athletes, in part because there is so much about the drugs they still do not understand. Studies have found that 25 percent to 39 percent of the weight loss comes from muscle loss. And because the drugs limit appetite, they could lead an athlete to underfuel. Especially in sports that incorporate weight classes or subjective judging, scientists and coaches alike fear that GLP-1s could encourage disordered eating.
Some patients have also reported a decrease in interest not only in food but also in alcohol and sex; Stuart Phillips, a kinesiology professor at Ontario’s McMaster University who has studied GLP-1s, theorizes that anhedonia, the loss of interest in usually pleasurable experiences, might also show up as a lack of enthusiasm to train at one’s peak capacity.
“Could [the drugs] take the top end off your strength or your VO2 max, your peak oxygen uptake?” he said. “We don’t know the answers to those questions.”
Especially because for the most part, scientists and elite athletes are studying different things.
A typical person might need to drop 10 percent of their body weight before noticing a difference; a professional runner might only need to shed a pound or two.
“They’re looking for tiny gains,” said Michael Joyner, an expert in exercise science at the Mayo Clinic who emphasized that he was speaking on his own behalf. He theorized that they might get there by microdosing GLP-1s, taking just enough of the drug to achieve the weight-loss benefit but not so much that they destroy their training.
He added, “The athletes and dealers who use these drugs typically know way more than the pharmaceutical industry or the physicians.”
Many of them are already on to the next iteration: Future versions of GLP-1s might be able to help patients maintain muscle mass.
The scientists are hurrying to catch up. Cowan and his team are, they hope, some two years from being able to detect some of the novel drugs using a new strategy: They look for a marker that a compound is synthetic, rather than testing for each compound itself. (He was reluctant to go into too much detail, for fear of tipping off the dopers.)
Meanwhile, the conversation only gets louder and more urgent.
“Athletes are currently using them,” Phillips said. “And they will probably begin to use them more and more until somebody tells them that they can’t.”
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