getting a doctor test soon

well im kinda asking for some help here i geuss, just wondering if ill be able to be clean for a test done by my doctor, not sure the specifics about that but im kinda an off and on smoker as in when i have it i usually smoke out all the time but its been about 3 weeks since i last smoked then i smoked yesterday and today about 5 times total, i have about 20 days i say before the test, im 5'7" and about 130 pounds as well but that fluctuates often, i smoke cigarettes, and do other drugs but am not worried abou tthem coming up, but if i stopped say with my last time being tomorow would i make it you think ? i have a very fast metabolism as well, which my friends have stated that i shouldnt have any problems cuz im very skinny, but im nervous, i've smoked quite a damn lot in my life but this is my second test ever, i failed my last one oops, but if i fail this i may be going into a serious rehab thing cuz i agreed with my doctor to stop when he knew i was addicted to cocaine but yeah so im thinking ill be going away if i fail this one AHHHHHHHH