Are you including alcohol and cigarettes and caffeine here?
EG: Ban any "unhealthy" food from being purchased using public assistance. Require anyone who is on public assistance provide proof of doing 30 minutes of daily exercise. Tie public assistance aid to your credit score. Place monitors in the house so if one is engaging in too much time-wasting activity (video game binges! hours of social media!) your funding is cut.
This could get pretty nightmarish frankly in an Orwellian way, especially if we combine this with the "world is so automated a basic income is absolutely necessary" idea that's floated around here a lot.
Generally, in the states that drug test for "welfare", the results seem to mostly point to a waste of money (typical articles on the phenomenon are here: http://www.forbes.com/sites/judystone/2015/02/17/the-sham-of...). As my above point on reagents, the standard "pee test" involves similar principals; the only way to truly confirm a positive pee test is to send it to a lab for GCMS analysis. This is not cheap.
Also, I don't really the see problem with a "nanny state" in the context of people that are accepting money from the government. If you need the government to provide for you, you should be totally happy with whatever stipulations that help includes.
I could use similar harsh, if negatively over-generic, logic to justify the rule ("I'm not paying for the health costs of some fat ass who ate so many donuts in their life they're now having a heart attack on my dime!"). But such an approach will probably not solve very much anyways except make a whole lot of people angry. People would probably hold donut-eating parties in protest. :)
One other point. At present, the current state legislative trend on drug-testing "welfare" recipients is mostly limited to TANF, with occasional forays into Medicaid and SNAP. I have seen no proposals targeting SSDI / Social Security or Medicare. There's a reason I said "caste" -- right now, the context is not people in general that are accepting money from the government, but only the "wrong" people that are accepting money from the government.
Oh, how many times I've heard the same thing, except referring to opiates, cocaine, or amphetamines, rather than caffeine.
I feel like the best thing to do in regards to drug legalization is to try to empirically link each class/type of drug to various societal outputs, like productivity, price of medical care, etc, and then tax the drugs accordingly.
Meth decrease general productivity by 100%? Tax it at a higher rate. Cocaine/MDMA/LSD have benefits and fewer side affects? Tax them at lower rates.