I don't think Sudafed is an antihistamine, that's not the reason she gave you a hard time (maybe commercial preps add an anti-histamine).  It is a pure decongestant but contains Pseudoephedrine (a vasoconstrictor, decongestant).   That's where the problem occurs.  Apparently, Pseudoephedrine (which by itself is fairly innocuous) can be used to synthesize crystal Meth.  Sooo..they are keeping "track" of who buys Sudafed and in what quantity.  The company which manufactures Sudafed has now substituted Phenylephrine for Pseudoephedrine in a product called Sudafed PE to avoid this obstacle.  Phenylephrine is the exact same stuff you get in nosedrops and will work just as effectively.  The only people that need to be cautious with these types of meds are people with high blood pressure, cause the constrictor action of the med does tend to raise the BP a bit.