Agree with David. Too many people take the naive view and say that anything experimental(i.e. numb, daddys gonna pay, babyface,your blue room, all of pop) must be a result of drug use. The reality is exactly as David said, there is no way a band like U2, who is expected to go to the top with all of the work they put out could be making albums on drugs. Some people are just in denial and actually think that drugs give them more insight, more creativity, etc when the exact opposite is true.
Drug use, whether you are making music, drawing a picture, driving a car, reading a newspaper, having sex, anything you can think of in life, will slow you down and distort things neither of which allow for performing any of these tasks well. Your capability at anything is greatly reduced if you are on drugs, despite what the cheech and chong crowd would have you believe.
Edge tried mushrooms, that story has never been confirmed/his words were taken out of context in the first place. Adam got busted w/ a joint in Dublin in 1989, which in no way represents a trend or pattern, he regretted breaking the law, was honest about it, said himself it was stupid, got completely sober after zoo tv, etc. None of them have been drug addicts for sure, they have never come out fucked up on stage like so many others do, they have too much respect for the music and their fans to do that. U2 is for all intents and purposes, a drug free and anti drug band(listen to Bad or running to standstill closely) and that comes from them seeing first hand what drugs did to their fellow Dubliners in the 1970s and 1980s.
U2 is where they are in part because they realize that drugs harm rather than help them as artists. If they used drugs at all, never mind to the point where it showed on one of their albums selling millions of copies, it would have come out, rest assured. Especially if people think the period in question is the 90s- they were already beyond celebrities at that point. No other band has stayed in the spotlight and out of trouble for 32 years, none, it is unprecedented and speaks for itself.