Look up "Drugs, views on" under the guys' names in the index of U2 by U2.
I honestly don't think that any of the guys have secret drug addictions. Sure, they've probably tried some drugs at one point or another, but they are way too sane and healthy to be -- or ever have been -- heavy users.
Look at Adam as a sort of barometer: he looks older than the other three because of his "wild" life (which probably wasn't as wild as other rock stars' lives) that mostly involved smoking a lot of various things. Now if Bono, for instance, was half as involved in that sort of thing, wouldn't he be looking more like Adam? (I'm not saying Adam looks really old, just older than the others.)
In other words, if they've been doing a lot of stuff, it would be showing by now.