The danger.
Show me one meth, crack, or heroin user who didn't smoke pot first. I'm not saying that all pot users move on to harder drugs, just that all hard drug users started with pot. Chasing the next bigger and better buzz is what draws them in. They don't call it a gateway drug for nothing. Come down hard on the growers, trafficers, dealers, and promoters. Maybe even design a desease that targets the weed, kind of like dutch elms disease or corn blight.