Caster Semenya will have to take medication to lower the natural levels of testosterone in her body if she wants to keep competing in the women's 800m.
It means that this weekend's Diamond League meet in Doha could be her last for a while.
It will not be the first time Semenya is subjected to medical intervention with the sole aim of curbing her testosterone levels, with the IAAF having enforced the same ruling back in 2012 before it was scrapped in 2015.
Semenya's times suffered badly during that period and she was nowhere near the dominant force she is over 800m today.
While the 28-year-old has been defiant on social media following Wednesday's decision from the Court for Arbitration of Sport (CAS) that the IAAF ruling will stand, Semenya will know better than anyone that this is a potentially killer blow to what was turning into one of the great athletics careers.
Depending on whether or not the IAAF accept the recommendations from the CAS - they do not have to - Semenya could still find hersel running the 1 500 m without medication.
As it stands, though, the IAAF ruling on testosterone limitations does include that distance.
As sports scientist Dr Ross Tucker said on Thursday, Semenya has three options: retire, medicate or turn her attention to the 3 000m and 5 000m distances.
With the Olympic Games set for Tokyo next year, Semenya will likely have to make her decision sooner rather than later.
In an interview with Sport24, Tucker helped explain what the medication and lowered testosterone levels might mean for Semenya.
"Every single system in her body that is normally affected by testosterone will go in reverse, as it were," Tucker explained.
"Testosterone helps increase lean muscle mass, so her lean muscle mass will go down. Testosterone means less fat mass, so her fat mass will go up. Testosterone means more haemoglobin, so her haemoglobin levels will drop. Testosterone is also really important for recovery from exercise. It is an anabolic hormone that helps build the body up, so she would lose the ability to recover from training.
"The collection of those four things means that she will probably see a negative impact on performance because of taking the medication.
"That is separate, by the way, of the side effects of the medication which could negatively impact her performance, independent of testosterone levels."
What this would mean in terms of Semenya's times is unclear, but it is widely accepted that the medication could see her 800m time dip by as much as seven seconds.
Semenya's legal team does have 30 days to appeal the CAS recommendation.