Performance diminishing drug 

The opposite of a performance enhancing drug, like marijuana.

That guy smoked pot before the race and raced like a turtle. It was a performance diminishing drug somebody on the other team told him would make him faster.

Performance diminishing drug by The Original Agahnim July 30, 2021

Performance diminishing drug 

A drug that does the opposite of enhance performance, such as marijuana.

Someone from the other team told the guy that if he smoked ganja before the race, he would be more clearheaded, faster, stronger, and to his suprise, it took him most of the day to finish. Nobody told him that marijuana was a performance diminishing drug.

Performance diminishing drug by The Original Agahnim July 30, 2021