job interview?

ok so i dont want to jinx anything so im knocking on wood but i think i might get this job interview and i was wondering are you drug tested upon interview or do you get drug tested later? Any tips to pass a drug test because i heard many things run and get sweaty and other things anything anyone had experience with? and im not dreaded yet but im going to get dreads and i was wondering if i get the job should i inform my boss that im getting dreads or just get them and show up to work? knock on wood again