Even at the regional level you get random testing and interestingly in its annual report last year British Cycling, which oversees all competative cycling events in Great Britain under the rules of the UCI did over 1200 random drugs tests and found one positive result for canabis, so either people are very good at hiding it and BC is complicit in helping people to hide it or there is nowhere near as much drug taking as people assume.
One of the reasons is there is a huge list of banned substances published by the UCI and if you get tested positive for any one them then you get a years ban so your season is lost you get your name and address published on the BC website and no club will touch you with a barge pole.