I quit drinking years ago and never used a single mind-altering substance during the pre-clinical years. I also managed to stay sober while studying for Step 1. What a crock.
I'll spare you the lecture about grass and you should know that 4 weeks is more than enough time to clear a single joint.
If you plan to practice medicine, however, I'd suggest that you grow the hell up. From the first day of 3rd year to the end of your career, you'll be subject to random drug testing at any time - certainly subject to drug testing for cause at a minimum. Sure, there are a lot of doctors who drink too much and who abuse other substances. And the screws are getting tighter every year. Is that really how you want to live your life - viewing all your colleagues with suspicion and wondering when the ax will finally fall? The days when staff were afraid to confront the doctor-God are long since over. Go read your state board's enforcement actions. First morning you show up with overly-bloodshot eyes, even if you're functioning perfectly, some nurse on your unit is going to be placing a phone call that you'd rather she didn't.
All right, rant over. This isn't an issue of judging you as a person, but I'll certainly condemn your behavior. Seriously, honest to God - if you felt the need to smoke while studying for Step 1 (knowing full well you might have a drug test sometime afterward) you are NOT ready to deal with the stress you're going to experience in 3rd year - not to mention residency. Get some healthier coping mechanisms - you'll need them.