Athletics starter Frankie Montas has been suspended for 80 games after testing positive for the banned performance-enhancing substance Ostarine, MLB said Friday.

Montas, 26, was in the midst of a breakout season in which he was 9-2 with a 2.70 ERA and 97 strikeouts against 21 walks in 90 innings. He will not be eligible to return to the A's until the end of September and will not be eligible to pitch in the postseason.