When does the pre-employment drug screen take place?

Hi all,
Just a quick question since I've never had to take a drug test for a job...when in the hiring process do most companies have you take the drug test? I have already received my offer letter and have gone through the orientation so I'm a bit confused and thinking that there is a SMALL chance I won't even have to take a drug test (since it is for a part-time position as a researcher at a university). I am not living in the place where the job is so I'm not sure if that may have something to do with it, although it is my understanding they usually have you go to LabOne or a place like it and have the results sent to the employer anyway. I'm planning for the worst case scenario and have been clean for 6 weeks now, still testing positive for thc when using home tests. I am also planning to do Jerry G's method in case of a hair test and cut my hair very short 2 weeks ago and shaved my entire body. I was a heavy user and could stand to lose a bit of a beer belly that I have. Any information would be good since I'm basically going into this blind. Thanks ahead of time!

It has been my experience that they test you before you actually report to work, Did you sign something saying your job is contingent on a Drug Screen? I would continue to expect one for the time being.

The job offer letter said that the job was contingent upon transcripts proving my academic standing, and that I shall comply with university drug-free workplace policies (which I have read and re-read and found nothing about pre-employment screenings, but instead having to disclose any criminal drug involvement within a few days of that occuring and having to be tested if injured on the job). I had signed a form basically stating that I would not do drugs while working there but that is it. This is why I'm a bit confused because I was under the impression damn near everyone tests these days for employment, but I have already gone through the orientation paperwork and have communicated with people there for over a month and nothing has been mentioned yet...

*bump*
alot of viewers and I know I'm not the only one has gone through the hiring process. any insight would be greatly appreciated...

It really varies depending on employers. Had a friend I talked to a couple days ago that didn't get his pre-employment test for a truck driving job till yeaterday. And he had been working for the company for almost 10 years.lol