Just make buying drugs like weed legal then you will not have a problem. They have in certain states but just do it for all states then you will have one less problem to worry about.
Illegal drugs will never be legal unless the state will say that it is just going to be legal for medical purposes. But it seems that drug trade with bitcoin is one of the hardest thing that the state to fight against. Because of its anonymity the people who are involved with it are going to have easy trades while not being caught.
Yes it is hard to eradicate drug trade if the currency used is bitcoin. The user and buyer may not know each other but only send wallet address and the drop point of the contraband or merchandise. But if this trade will continue and will be on a large scale then possibly it is another bad news for bitcoin. The government will interrupt bitcoin uses as to eradicate illegal drug trades, and the state may issue an order banning bitcoin to kill illegal drug trade activities in a certain country. Hope the users will not buy drugs with bitcoin so that the state will not interfere with cryptocurrency.