Star boxer Ryan Garcia tested positive for the performance-enhancing substance Ostarine the day before and the day of his upset win over Devin Haney last month, per a Voluntary Anti-Doping Association letter sent to all parties Wednesday and obtained by ESPN.

The samples were taken prior to the fight, but the results weren't known until later.

Garcia has 10 days to request that his B-sample be tested. Garcia's A-sample also screened positive for 19-Norandrosterone, but its presence is unconfirmed at this time.

"Everybody knows that I don't cheat," Garcia said in a video posted on X. "... Never taken a steroid ... I don't even know where to get steroids. ... I barely take supplements. Big lies, I beat his ass."

Garcia, 25, floored Haney three times during the majority decision victory, but that result now stands to be overturned unless the B-sample returns negative, which is rare.

"We learned about this situation not too long ago and it's unfortunate Ryan cheated and disrespected both the fans and the sport of boxing by fighting dirty and breaking positive not once, but twice," Haney said in a statement to ESPN.

"... Ryan owes the fans an apology, and by his recent tweet he still thinks this is a joke. We put our lives on the line to entertain people for a living. You don't play boxing. This puts the fight in a completely different light. Despite the disadvantage, I still fought on my shield and got back up! People die in this sport. This isn't a joking matter."

Garcia also missed weight ahead of the fight, registering 143.2 pounds for the WBC 140-pound title fight. Haney, 25, remained the junior welterweight champion even in defeat.

Ostarine is a selective androgen receptor modulator which attaches to proteins in the body and effectively tells muscles to grow. It is used to aid performance by helping athletes build muscle mass and enhance their rate of fat loss and also to increase staining and recovery ability.

It's been on the World Anti-Doping Agency's banned list since 2008 and in 2022 was listed as an anabolic agent by WADA.

Ostarine has been used in boxing before. Lucian Bute tested positive for it in 2016 following a draw with Badou Jack in their WBC super middleweight title fight. The result was changed to a DQ win for Jack.

Amir Khan was handed a two-year ban by UK Anti-Doping (UKAD) after he tested positive for Ostarine following his sixth-round TKO loss to Kell Brook in 2022.