For me, this is where it matters whether or not it's for sale. If you're growing your own opium poppies, cooking your own meth, or whatever, I'm fine with that. Give some to your friends, even.
If you start selling it, or make it a business, then it becomes everyone's problem, and government should step in. If/when weed is legalized nationally, the big tobacco companies (plus a few others) will smell money, take over the market, and advertise like crazy; then we'll have to set some limits.
Drugs have to be regulated on a case-by-case basis. Caffeine is available to anyone and untaxed. Alcohol is only available to those over 21, and taxed. Tobacco, 18 and heavily taxed. Opium? I'm not comfortable with selling opium becoming a legal business model. Like most things in life, drug legalization is a compromise.