Florida recently passed a law requiring welfare recipients to be tested for drugs and throwing them off welfare if they test positive. Governor Rick Scott justified it as saving taxpayers’ money and discouraging drug use. It turns out to be costing taxpayers more money than it saves them, because hardly anyone tests positive. This isn’t conclusive proof, by the way, that the law isn’t discouraging drug use – after all, prospective welfare recipients could have modified their behavior after the law was passed – but it’s strongly suggestive that it is not, for low-resource citizens tend to have higher levels of political ignorance, and it would not be surprising if many of them did not know of the new law before applying.
Even if the law were working as intended, I think it would still be unjust. As Mike Riggs points out in the first link above, the law does not test corporate welfare recipients for drugs, only poor people. The fact that this is a government benefit does not mean that the government is justified in attaching any conditions it wants to it. Would it be justified in requiring every public school student (and parent??) to be tested for drugs? Would it be justified in requiring strip searches of welfare recipients? Drug testing is invasive and should always bear a significant burden of proof when conducted by government. In my view, while private employers have every right to test their employees for drugs, the bureaucratic, “zero tolerance” culture of drug testing has gone too far and should not be further encouraged.
PILEUS 
For that matter, why not drug testing to use government-provided roads (viz. driver’s licenses)? Once you start down that path, it’s hard to see where there is a principled stopping-point.