You will never get hired by the FBI after misusing prescription medication.
I studied Criminal Justice in college, it is one of the better CJ programs in the country, and the head FBI recruitment agent for my state came in to speak to my Federal Law Enforcement Class (which was taught by an active Chief U.S Marshal).
She handed us out several documents, one of which was an "instant disqualification" sheet. Basically anything above smoking week "a handful" of times will get you 100% permanently disqualified from all Federal LEO positions, the FBI especially as it is the most popular and most applied to.
The whole room was basically in hysterics because most people studying CJ dream of being a federal agent, and typically think local law enforcement is beneath them after studying for 4 years (when the hilarious reality is that most of them won't even be lucky enough to get a local job in this job climate). I believe the only other drug that wasn't an instant remove from the process was steroids, as long as they have not been used within 10 years. Needless to say, most college students have experimented with drugs, so there was a big depressed feel in the crowd. I think some people were even crying; that was the first time they really heard that rumor confirmed by the actual source.
They followed up with a story about a guy from my university who was a stellar candidate; One of the captains of the football team, 4.0 GPA in CJ, tons of community service work. He went in for the U.S Marshal's with a recommendation from that state's Chief Deputy Marshal (the professor), the job was 100% his, and yet he was dropped out the process. When the professor inquired about what the hell happened, because he really liked the kid, he told him that he was honest and admitted to using Molly once at a party after being peer pressured as a freshman on the team. Again, just once over 4 year prior; and he was dropped instantly.
It's almost impossible to be hired by the FBI, she said that for every 20 positions they open for Special Agent they get over 25,000 applications. So they can disqualify you for almost everything, and can be insanely strict with their standards. I heard the internships are just as competitive.
The bare minimum experience for agent positions as well was:
Bachelor's degree absolutely required.
Supplemented with 4 years work or military experience on top of that.
OR Supplemented with a graduate degree (MA, MS level) and at least 2 years work experience or military service.
OR Supplemented with a PhD., a JD, or MD.
And obviously no prior drug use, ect.