False positives DO occur…my family is living through the experience. My physician husband has a short lived history of substance abuse (6 weeks during an acute major depression)over 2 years ago. He is randomly tested (full toxicology) twice weekly. He was the first to be subjected to the random monitoring by the provincial medical board and the punishment was very harsh – it included suspensions of over two years while they figured out how to handle it.) He has not abused anything since Jan 04. However we have had our 4th false positive. The first was for cocaine (a drug he’s never been anywhere near) – likely due to an antibiotic. A negative hair test cleared him. This month we have three false positives for cannabis – likely due to high dose anti-inflammatories. Only problem is, we can’t find credible information on the interference of ibuprofen in the gc/ms process. He was taking very high doses of ibuprofen, then toradol for a massive dental infection and continues to take the stuff due to osteomyelitis of the mandible. He had a hair test done this week (results pending). Local “experts” say hair testing is not as good for cannabis. End of the story, he’ll likely be forced to surrender his license to practice medicine on Apr 26th because we can’t prove he didn’t do it.