Decriminalisation is precisely the form that alcohol prohibition took in 1920s America. It led then to the same problems that all-out prohibition causes now - increased murder rate due to turf wars, increased deaths because of impurities, widespread corruption and the diversion of law enforcement assets away from real crimes.
The only sensible solution is legal supply. The form of supply could vary depending on the substance involved - ranging from a free market for cannabis to prescription only for heroin, for example.
The Legalise Cannabis Alliance