Jannik Sinner, the world No. 1 ranked men’s tennis player twice tested positive for the steroid clostebol, but despite this the International Tennis Integrity Association (ITIA) announced Tuesday that he won’t face any penalty for the positive tests.

“A thorough investigation by the ITIA followed, including multiple in-depth interviews with Sinner and their support team, all of whom co-operated fully with the process. Following that investigation, and in line with the independent scientific advice, the ITIA accepted the player’s explanation as to the source of clostebol found in their sample and that the violation was not intentional.”

It’s one of the most stunning developments at the top of world tennis since Maria Sharapova was suspended for performance enhancing drugs in 2016. So how did Sinner, the top seed in the U.S. Open starting this month, manage to be cleared so quickly for a steroid? That comes down to the details behind the test.

Clostebol, also known as “chlorotestosterone,” is a synthetic derivative of testosterone with legitimate medical uses. The drug can be taken orally, or more commonly as an additive to a variety of ointments. It can be used to treat a variety of dermatological conditions consistent with other applications of steroids, but it’s here where problems are arising for athletes.

Sinner’s camp claims he was being treated by a massage therapist who used clostebol for a personal skin condition. This in turn transferred dermally to the athlete during treatment, and popped the positive test. The ITIA acknowledges that the amount of clostebol found in Sinner’s system was very low, indicating that it’s likely the story from the athlete is accurate and that he was not intentionally abusing the drug.

This is part of a multi-year problem that the World Anti-Doping Agency (WADA) has been dealing with when it comes to clostebol. A 2020 report indicated that accidental doping was highly-possible with the drug, especially in Italian athletes. The use of creams containing clostebol are prevalent in Italy, and despite carrying a WADA warning makes it easy for athletes to accidentally take part in doping, especially when those in their proximity are using the drug.

As for Sinner, all signs point to this being an honest accident caused without the athlete’s knowledge, and the WADA report of clostebol use in Italy backs that up. The low-levels of the drug found in Sinner’s system also support that this wasn’t an intentional or significant case of doping, which is why the ITIA supports moving forward without punishment.