We do this with Wellbutrin in correctional psychiatry. Inmates will sometimes try to abuse this medication to get high (crush, snort, take more than prescribed), and they can also have it stolen or trade it with other inmates. With a random blood draw and a (supposed) time of last administration we can look at the concentrations of both the drug and its metabolite; their ratios give us an accurate picture of what the inmate has taken for the past three days.