I don't think drug tests should be required for welfare. I'm totally against entitlement programs but the idea of a test would only make sense if it included nicotine, caffeine and alcohol.
If the argument is that we shouldn't have tax $$$, via welfare payments, going to frivolous items, then mind altering drugs fall into that category as well as any chemicals that are completely unnecessary for day to day life but are available and habit forming- caffeine and nicotine. If my tax $$$ shouldn't fund a welfare-case's drug habit, it shouldn't fund their Starbucks or Marlboro habit either.
If the idea is that drug use is harmful for society, and tax $$$ should not pay, via welfare, for substances that may cause problems, then it makes no sense to not include alcohol. I used to watch folks in section-8 apartments all day long walking back & forth to the bars past the gas station I worked at, buying 40's and 6-packs. What ticked me off to no end was there was a beer distributor 3 blocks away they could have bought in bulk for far less. And according to my friends in LE, they've never been to a domestic violence call where everyone involved was just smoking pot. But when alcohol is involved, that's when people start getting stabbed/shot/beaten to a pulp/killed.