Well, here is the States they do pre employment drug testing. Its done first and a clean result must come back before they will even continue with the interview. That being said, the junk you smoked is one of the things they test for, you come up positive on pre employment, well you are not going to get hired, once employed, they do random, you come up positive on that, you can be terminated. Most companies that do drug testing, and its not just transportation don't want to even deal with it, so you do, you don't work or not work at something you want to do. On your medical, here in the States they do ask, about it, if you get caught and don't inform the FAA, you Certificate goes away. Same for heavy drinking, they days of drinking breakfast, then go fly all day are gone.