Casey is right, ask the lab tech, your pharmacist or whoever is administering he test about that.   Just so you know, some over-the-counter antihistamine/decongestants contain pseudoephedrine, which is the drug that makes them work, but is also the drug that people use to make crystal meth -- an amphetamine.   I have no idea if taking Sudafed will cause you to test positive for amphetamines, so I would ask. In the mean time, you may want to avoid it if you're concerned.   The drug in Zyrtec is Cetirizine hydrochloride, not pseudoephedrine. So, theoretically, it shouldn't cause any positives, but like I said, I don't know that for sure.