In our case we had let our son use our cars lately, as he didn't show signs of drug use. He was going out, but he didn't drive home, which we found annoying from a sense he had our car out, but he has never driven under the influence, he actually has been very good about that. In his worst times, he simply didn't drive.
He has a strong view on driving under the influence and will cab home and leave our car downtown. Like I said, we find it annoying but are relieved he doesn't drive under the influence. I know you will say how can we be sure... but like I said, it's him, he simply won't drive if he feels he is high or drunk. We are fortunate in that.
His accidents have been bumpers, and have for three out of the four been not his fault. The last one, he backed up and didn't see something in the way. It isn't a lot of damage but our feeling is that he needs to pay for it.
We don't worry about the impaired driving with him, it honestly hasn't been an issue. But in the case we ever felt he was using heavily ( like he was a year back) we cut off all access to our vehicles.