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With her reentry into the pool for anti-doping testing, Williams becomes the most prominent athlete linked to GLP-1 drugs—as regulators weigh whether they belong in elite competition.

The Takeaway

• Serena Williams’ anti-doping reentry could raise scrutiny of GLP-1 use by athletes.
• The World Anti-Doping Agency monitors GLP-1 drugs for performance impact.
• Inclusion on the agency’s monitoring list doesn’t guarantee a ban.

An earthquake shook the sporting universe this week: Serena Williams, the 23-time Grand Slam tennis champion, has taken the first step toward a potential comeback by reentering the sport’s anti-doping test pool. After news reports surfaced on Tuesday of her registration with the International Tennis Integrity Agency, Williams tried to quell speculation about a potential return to play for the first time since 2022. “Omg yall I’m NOT coming back,” she posted on X.

Whether or not she ever plays another professional point, Williams is now the highest-profile athlete known to be taking a GLP-1 drug who has submitted herself to the testing pool. Her participation comes at a time when the sports world is actively evaluating whether drugs like Ozempic, Wegovy and Mounjaro should be allowed in elite competition.

The World Anti-Doping Agency oversees global anti-doping standards for scores of national and sport agencies, including the ITIA. Since 2024, WADA has included the GLP-1 semaglutide (a generic name for Ozempic and Wegovy) on its list of monitored substances, and it will add tirzepatide (generic for Mounjaro and Zepbound) beginning in 2026. The monitoring list is a kind of purgatory for substances that WADA may or may not choose to ban from elite sports in the future, but that require close study to understand whether and how they impact performance.

Though a drug’s presence on the monitoring list doesn’t guarantee an eventual ban, if WADA chooses to do so, it will provide athletes with ample notice. WADA reviews and updates its list of prohibited substances each autumn before the new list takes effect January 1, giving athletes time to either stop taking a particular drug or obtain a therapeutic use exemption. (Famously, Maria Sharapova’s positive test for meldonium at the 2016 Australian Open came after she admitted she had not read WADA’s prohibited list for that year, which banned meldonium for the first time.)

For WADA to designate a drug as a banned substance, it must satisfy two of the three following criteria: It has the potential to enhance performance, it risks the health of the athlete and/or it “violates the spirit of sport.”

WADA has studied some substances on the monitoring list, like nicotine and caffeine, for years without banning them. Other substances have been the subject of intense lobbying and politicking within the anti-doping world. Cycling, a sport with no shortage of its own doping controversies, has campaigned heavily to ban the painkiller tramadol from competition; it remains on WADA’s monitoring list.

Few classes of drugs have had as much immediate social impact as GLP-1s. As The Information reported earlier this year, a wide set of the general public—and especially the technorati—has adopted the substances, initially developed to treat diabetes, for weight loss and other bio-hacking purposes. It’s not surprising that athletes would use Ozempic or Mounjaro for similar reasons.

Williams has been incredibly vocal about her GLP-1 use. She’s the face of an advertising campaign for telehealth company Ro (her husband, Reddit co-founder Alexis Ohanian, is an investor in it). In that campaign, she claims she lost 31 pounds on GLP-1s in less than a year. When opening up to Vogue this summer about going on Zepbound, she said, “I don’t really care what people are saying about my body anymore. But what is important to me is transparency.” (Through a spokesperson, Williams and her agent did not respond to requests for comment.)

Williams last played professional tennis in 2022. Carefully avoiding the word “retirement,” she instead said she wanted to “evolve away” from the game she utterly transformed. Along with elder sister Venus Williams, Serena burst onto the women’s tour in the late 1990s as a bubbly teenager before eventually winning more Slams than any woman in the Open era. Many consider her the best to ever play the game, with nearly two dozen Slam trophies and four Olympic gold medals under her belt.

Williams is not the only celebrity athlete who is open about their use of GLP-1s; fellow tennis legend Billie Jean King and basketball star Charles Barkley have also discussed using injections to manage their weight. King and Barkley have long retired from sports.

Meanwhile, anti-doping authorities are studying whether weight manipulation tied to GLP-1 usage affects athletic performance.

For elite athletes, weight is a highly specific variable in the pursuit of athletic excellence. Boxers and weightlifters compete in stratified weight categories. Added muscle can help sprinters and basketball players perform better. Leaner frames can yield benefits for endurance runners or figure skaters.

So far WADA authorities have agreed there is not sufficient evidence to suggest GLP-1s are performance enhancing, Travis Tygert, CEO of USADA, told me in an email. In fact, he said, sport “performance diminished with muscle loss and other side effects.”

But the drug category still needs further study. Dr. Matthew Fedoruk, chief science officer of USADA, explained that there is a new class of drugs currently in clinical trials that, when taken with semaglutide, has achieved a kind of holy grail for athletes: weight loss with minimal diminishment of muscle mass. One such example is Enobosarm, also known as ostarine, which was developed to combat loss of muscle in patients undergoing treatment for cancer.

“It’s really important to keep a close eye on these new drugs as they could potentially be abused by athletes,” he said.