The 23-year-old -- who moved to Stoke from Premier League rivals West Brom in the January transfer window for £12million ($15 million, 13.9 million euros) -- claimed his drink was spiked at a night club.
"To be banned for something you really haven't done is hard to take," the former England Under-21 international told the BBC adding the level of the drug in his body was so low it proved he hadn't intentionally taken it.
"You go on a night out, you don't know who you are around and there are people who are out to get you.
"Of course, I was in a nightclub, so I hold my hand up for being irresponsible.
"From then on it all crashed down. To this day, I still cannot understand who would want to do that to me."
Berahino, who caught the eye in the 2014-15 season when he finished as West Brom's top scorer with 20 goals in all competitions, didn't play for his former club from September 10 last year to the day he left.
His and the club's relationship never recovered after he fell out with West Brom manager Tony Pulis and chairman Jeremy Peace after being denied a move to Tottenham Hotspur in August 2015, resulting in a long-running contract dispute.