If they are fully complaint with WADA (which I believe is the case) then recreational drugs on competition day will get you disqualified.
Most non-weed drugs clear the system in 24 hours, weed can be a week. I believe if you fail a test, you can not argue that you took something the night before (ie. not on competition day)
Some sports test for recreational drugs outside of competition but this is pretty much unheard of in strength sports.