My observations as a controller, is that we as a group take the job too seriously to ever turn up to work under the influence of any drug, alcohol or otherwise. Of course caffiene and nicotine are notable exceptions which are in fact a neccesary part of the preparation for a morning or night shift, myself included!
I think drug tests would be an unneccesary invasion of privacy, unless of course there is a big screw up, then as in a car crash, it would be expected that a urine test would be conducted. Blood tests to find traces of drug use from 90 days earlier is frought with danger, as the individual may have been in Amsterdam as an example and quite legally indulged in a bit of "choof", yet have their career put at risk.