Being closely associated with the tube i would like to say that since the introduction of random drug _and_ alcohol test the incidence of abuse, of alcohol at least, was cut, almost overnight. I'm not sure how prevalent drug abuse was.
Anyone is subject to random testing. If a manager suspects abuse by a member of staff they can arrange to meet the Transport police at a location to test said member of staff.
Personally i support this policy i have long waited to see it introduced into aviation. I believe it is not just the airlines that require it but instructors. From my happy ppl learning days i probably shouldn't have gone up with my instructor on occasions.
I can't remember whether this was highlighted during a hostestess' BA pilots drinking 5 minutes before T/O but one of the old eastern block countries test every pilot before they board?