"I can confirm reports that Sbu underwent a World Rugby test in May and his A sample was positive," said the official, who requested anonymity as he is not the club spokesman.
The 28-year-old winger, who understudied Cheslin Kolbe at the World Cup in Japan, could be banned for four years if his B sample is also positive.
Another 2019 World Cup-winning Springbok, fly-half Elton Jantjies, was suspended for four years last January for using the banned substance Clenbuterol.
Nkosi won the first of 16 Test caps against England in 2018 and his last appearance was against New Zealand three years later.
He suffered mental health challenges after moving from the Sharks to the Bulls and disappeared for several weeks in 2022, triggering a police search, before being found at the home of a relative.
Cheetahs, who gave Nkosi a one-year contract this year, withdrew the winger from a Currie Cup match against the Bulls in Pretoria on Friday, citing "personal reasons".
