coke laced weed happens more than you would think. I work at a hospital where people get screend before surgeries with a urine test. And those that claim (and convincingly) to have only smoked weed recently, but have never done coke, come up surprisingly often with coke in their system too. Im sure some like about having done coke, but its too often for them all to lie about this shit. I wish I had numbers from my work to back it up...but trust me... it happens more than you want to think.
and yea, the use of prescription, rather than illicit drugs is a pretty week argument in most respects. its true you know whats in them, but... Those drugs are only deemed safe (by studies) in very specific intake amounts... all bets are off the table when someone is taking them in quantities far greater than recommended, or by a route other than intended.