The IAAF has confirmed new regulations over hyperandroginism, which will see females like South Africa's Caster Semenya, whose testosterone levels are above a certain level, take medication if they wish to continue participating in certain events.
These new rules will affect South Africa's double-Olympic champion, with the IAAF putting testosterone quotas on events from 400m to the mile -- which include Semenya's preferred 800m and 1500m distances.
Under the new IAAF ruling, athletes who have what they term a 'Difference of Sexual Development (DSD),' will need to take medication to lower their levels to fulfil certain criteria.
The rules stipulate that athletes with DSD need to be, "recognised at law either as female or as intersex," and "must reduce her blood testosterone level to below five (5) nmol/L for a continuous period of at least six months," and then, "maintain her blood testosterone level...for as long as she wishes to remain eligible".
If female athletes affected by the ruling do not take wish to take the medication, then they will still be allowed to compete in long-distance races, in non-International Competitions in those events now restricted, or in the male classification.
These rules will be activated on Nov.1, 2018.
"Like many other sports, we choose to have two classifications for our competition - men's events and women's events. This means we need to be clear about the competition criteria for these two categories," IAAF president Sebastian Coe said.
"Our evidence and data show that testosterone, either naturally produced or artificially inserted into the body, provides significant performance advantages in female athletes.
"The revised rules are not about cheating, no athlete with a DSD has cheated, they are about levelling the playing field to ensure fair and meaningful competition in the sport of athletics where success is determined by talent, dedication, and hard work rather than other contributing factors."
The decision will likely provoke outcry from some corners of the athletics world, with Yale Professor Katrina Karkazis previously saying the approach was blinkered.
"Semenya's athleticism was attributed to a single molecule -- testosterone -- as though it alone earned her the gold, undermining at once her skill, preparation and achievement," Karkazis wrote in the Guardian in 2016.
The debate over hyperandrogenism was in the headlines back in 2009 when Semenya won 800m gold in the World Championships. She was subjected to a gender test, while some of her competitors started questioning her times.
The IAAF looked into the effects of raised testosterone levels on performance in 2011 and their findings recommended an 'effective therapeutic strategy' for those whose levels were above a certain quota.
Semenya's times were seemingly effected by these new measures but she still took gold at the London Olympics in 2012 after original 800m winner Mariya Savinova-Farnosova was later disqualified by Court of Arbitration for Sport after being found guilty of doping.
But in 2015, CAS suspended the IAAF regulation after Indian sprinter Dutee Chand challenged the governing body, saying the tests were flawed. CAS responded by challenging the IAAF to prove raised testosterone levels had a direct impact on performance, putting a two-year hold on the regulations and allowing Semenya to compete medication-free. The South African comfortably won 800m gold at the 2016 Rio Olympic Games.
A year later, a medical study commissioned by the IAAF was published, which said higher testosterone levels gave female athletes a "significant competitive advantage" across the 400m, 400m hurdles, 800m, hammer throw and pole vault events. These were presented to CAS, but it delayed a definitive decision.
In March this year, the IAAF announced plans to introduce new testosterone quotas, with these confirmed on Thursday.
"The latest research we have undertaken, and data we have compiled, show that there is a performance advantage in female athletes with DSD over the track distances covered by this rule," said Dr Stephane Bermon from the IAAF medical and science department.
"We have seen in a decade and more of research that 7.1 in every 1000 elite female athletes in our sport have elevated testosterone levels, the majority are in the restricted events covered by these regulations."
Semenya has stayed quiet on the matter of late, but did suggest she was looking to longer distances after completing Commonwealth Games gold across the 800m and 1500m this month.
"I'm still 27, when I do my long runs I feel like I can still fit into distance running. So for me, this is more than a game," she said.