Not to say I agree or disagree with taking drugs. However, if the argument is, we wouldn't have discovered drugs if God didn't approve of them, how can we be sure? Plenty of other temptations also exist in this world that are prohibited. Surely, a reasonable God wouldn't be giving a person peadophillic desires and allowing child porn and be ok with people indulging in all that.
The argument that God only allows nice things doesn't really stand up. I get the impression more that God (if there is one) has filled this world with wicked temptations, made us very weak to temptation and then, let us loose! Probably laughing at the result. Who's to say God isn't a sadist who enjoys punishing people? That makes more sense to me. Why else would we be so enticed to do so many of the 'deadly sins'? It's like we've been set up to fail.
It also seems to me like a good measure that- things are only really harmful if they aren't hurting others. Is drug use and specifically addiction entirely harmless though? Surely, there are a lot of violent crimes committed because of illegal drugs. Is it truly a victimless crime? Do relatives enjoy watching their loved ones slip into addiction, poverty, destitution? Which I imagine can happen if they can't afford their habit but, can't work either. I imagine drug use can hurt multiple people- sadly, if it gets out of control.