UNITED NEWS INTERNATIONAL (UNI) — The international governing body for athletics, also know as the IAAF, is changing its rules for female competitors with naturally high testosterone levels.

As of Nov. 1, female runners with a “difference of sexual development” will have to either race against men or take medication to lower their testosterone levels.

High testosterone can increase muscle mass and strength.

The rule, which was announced on Thursday, April 26, will apply to women who race in track events from 400 meters up to the mile, including hurdle races.

The IAAF says the decision will level the playing field and will stop women with high testosterone from gaining an unfair advantage.

Africa’s 800-meter Olympic champion Caster Semenya, is expected to be one of the athletes affected. Semenya, however, could move up to longer-distance events if she doesn’t want to take medication.

Meanwhile, many South Africans took to social media on Thursday to express their anger over the rule change.