A lot would depend on my relationship with the person and whether the person had a job where drug use would be dangerous.

If I were pretty good friends with the person, I'd talk to them, let them know I know they use drugs for recreation. If they also had a job with machinery where they could hurt themselves or others I'd let them know I felt I should tell their supervisors of their non-prescription drug use. However, if there was random drug testing in place for the job (which there should be if it's a job of that sort), I might not mention telling but I would mention the drug tests (to ascertain if they were cheating in some way and trying to get around them).

If I didn't know them very well and had not seen them using drugs consistently and/or there was random drug testing in place (and I hadn't observed them enough to see their pattern of drug use) I'd probably leave it alone.