Prediction markets such as Kalshi and Polymarket—platforms where users trade contracts tied to real-world outcomes—are increasingly being used for sports betting, and athletes are warning that the platforms carry negative—and even violent—consequences for players.

Speaking at the Fortune Most Powerful Women Summit in Santa Barbara, California, on Tuesday, Nneka Ogwumike—the former Los Angeles Sparks forward, 2016 league MVP, and 11-time All-Star—said prediction markets are having a clear adverse impact on WNBA players.

"It's not a safe place for athletes," Ogwumike told Most Powerful Women editor Emma Hinchliffe. "I don't think I ever want to say that something is outrightly bad unless it absolutely is, but players are definitely having a hard time dealing with the vitriol that comes with prediction markets as it relates to our performance."

Prediction markets have emerged as an alternative and supplement to online sportsbooks because they are classified as financial exchanges rather than gambling platforms, allowing them to bypass regulations and restrictions that apply to traditional online sportsbooks. Last week, both Kalshi and Polymarket hit new records, according to Dune Analytics data, reaching a combined weekly trading volume of $22.2 billion, driven by NFL- and NCAA-bolstered activity—scale that underscores how deeply sports outcomes have moved into mainstream financial trading.

The concern is not hypothetical: harassment tied to sports betting extends to college athletics as well. The 2025 NCAA Growth, Opportunities, Aspirations and Learning of Students in College study found that more than half of Division 1 men's basketball players reported social media abuse based on their on-court performance, and 46% said the highest rate of negative or threatening messages came from someone who had a bet on a game they played in—data that gives athletes' warnings a documented foundation.

On Tuesday, the NBA and WNBA announced a responsible gaming campaign, partnering with the National Council on Problem Gambling (NCPG) to educate fans about the risks of sports betting. How that campaign is received, and whether regulators revisit the exchange classification that lets prediction markets sidestep gambling rules, remain open questions to watch. Ogwumike said more efforts are required to mitigate abuse of players connected to betting.

"There's been a lot of safety issues and concerns that haven't necessarily been publicly displayed," she said. "Through the [players association] and through the WNBA, there's resources that are available to try and mitigate those things, but it doesn't mean that it stops players from experiencing things that could potentially and have potentially affected their performance."

This story was originally featured on Fortune.com.