I think your question in a way anticipates my answer.
We've seen a big investment in enforcement and interdiction. CSC's own research, their internal audit report, etc., has demonstrated that there is in fact a bit of a plateau, and interdiction numbers seem to be going up. So there could be more seizures, but we really don't know if that increase in seizures has decreased the supply of contraband inside institutions because we don't have a baseline.
We do know about urinalysis, we do know about hepatitis C and HIV infection rates, and we know that these things are maintaining some stability over time. There's some suggestion that drug use seems to have levelled off in spite of the large new investments in enforcement and interdiction.