Caster Semenya has lost a landmark legal challenge against athletics’ governing body, meaning she will have to take testosterone blockers if she wants to continue competing.

The IAAF wants the South African and any other female athletes with what it calls “differences in sexual development” to take medicate to combat an “unfair advantage”. 

The double Olympic champion appealed the IAAF’s decision, saying she wants to “compete naturally”.

But the Court of Arbitration for Sport (CAS) ruled on Wednesday that the International Association of Athletics Federations (IAAF) regulations are needed to ensure fair competition between athletes.

The CAS agreed the regulations are discriminatory, but said “such discrimination is a necessary, reasonable and proportionate means of achieving the IAAF’s aim of preserving the integrity of female athletics in the Restricted Events.”

It means that Semenya and other affected athletes hoping to compete at the World Championships in Doha in September would have to start taking medication to lower their testosterone levels to below the required five (5) nmol/L within one week.

They must then maintain it below that level continuously, Reuters reports.

Semenya reacted to the news with a Twitter post that said: “Sometimes it’s better to react with no reaction”. The athlete added that her lawyers are considering an appeal. 

Testosterone is a hormone that increases muscle mass, strength and haemoglobin, which affects endurance.

Semenya’s dominance in middle-distance races has been labeled unfair by many of her competitors, but the proposed restrictions have also been branded as discriminatory.

Several expert witness were called by both sides during a five-day hearing presided by three judges at CAS’s headquarters in Lausanne, Switzerland in February.

Last month, Semenya said in a statement released through her lawyers that she wanted to differentiate her case from those of transgender athletes who were born male.

“Ms Semenya is a woman. There is no debate or question about this and the IAAF does not dispute this,” the statement said.

“She was born a woman, raised a woman, socialised as a woman and has competed as a woman her entire life.

“Ms Semenya does not wish to undergo medical intervention to change who she is and how she was born. She wants to compete naturally.”

The IAAF has previously said it is confident of the legal, scientific and ethical basis for the regulations and expects CAS to reject Semenya’s appeal.