Good idea in my opinion, but implementation without damaging the children of druggies is going to be difficult. Not really sure how you solve that issue. I have no problem with the idea that positive drug test = no government support, but implementation is close to impossible.
One thing I would like to see done is that the standard for welfare would be a requirement for a GED or a high school diploma. Obviously you could appeal the process, but we need at some point to make some dents in the systemic problems of welfare. Kids right now grow up with the parents getting their monthly check, and so they do the same. If school gets tough they drop out, live off their parents until they are old enough, and then start collecting their government check. It happens all the time (if you don't believe it happens, come to Mississippi and I will show you). There is no simple answer to this problem, and it has been caused by a lot of factors that largely aren't the impoverished people's fault, however we are providing motivation for them to stay on welfare. It is easy, they know they can live off it, they feel safe with their guaranteed income.
If you implemented an educational requirement on receiving standard assistance, the teachers would have more power in schools to demand excellence. Suddenly being kicked out of school creates GRAVE consequences. One could always go back, get their GED and then receive welfare, but that is not always the easiest thing to do. We need to get to a point where we are encouraging self improvement, advancement, instead of giving motivation to maintain the status quo.
This drug-screening idea isn't a bad idea, it is just hard to implement. I think an educational requirement would be much easier to implement because you could simply start it with people being born in 2015. Set a start date, so that generation would know as they grew up that welfare would not be offered if they didn't graduate. Take away the fallback, and people get a lot more motivated to perform, to learn, to advance. There is absolutely no reason that given the proper motivation a child can't graduate from high school or obtain a GED.
Anyway - just my two cents. I rabbit trailed a bit, but in general...drug testing not a bad idea, not sure how it works in a real day implementation.