I saw where a number of states are considering mandatory drug testing for welfare benefits. While there may be some logic behind the idea, it is ultimately bad public policy. The problem lies with the precedent it will set for others.
In other words, if you are drug testing someone who receives a government check, then shouldn't all government employees be drug tested? But some would argue that this is a government transfer, and not a government payment for services. Fine, then how about the millions of college students who receive federal and state grants every year? Logic would follow that they should be drug tested as well.
I saw that the ACLU has already filed lawsuits challenging mandatory welfare drug testing. It's funny, when I was younger I always looked at the ACLU as a group blocking reasonable legislation. Now some years later (and after studying economics), I tend to agree with the ACLU more often than not. This is mainly because they look at how precedents will affect other third-party groups, besides those groups who are directly involved.
As others have said, a lot of this welfare drug testing is about finding a scapegoat. It's a bad idea, and it's bad public policy.