As someone who's taken more than her share of drug tests for work reasons (pre-employment and random tests, not because they suspected anything
), when it comes time to take your test:
1. Be sure to tell them EVERYTHING you've taken in the last couple of weeks, even if it's a OTC or herbal medicine. Leave nothing out, not even those Flintstone chewables. Don't worry about being judged--it's not their business why you're taking what you have to take. Besides, most places are usually looking just for illegal drugs and not everything you're taking (but don't hold me to that--check with your school for the final word).
2. If you're on prescription medication, bring a copy of the prescriptions if you can, or at least the bottles. You could also have your doctor write a note verifying that you're on these medications.
3. If you think one of the medicines that you're on will possibly/definitely have an adverse effect on your test, talk to them BEFORE the test. It is a lot less trouble for both you and the school if you're upfront about what medicines you're on before the test, than trying to address an unexpected false positive result later. Trust me on that one.