Sudafed (pseudoephedrine or phenylephrine, depending on which variant you use) is a decongestant. You use them to clear up that stuffed-up sensation in nose, sinuses and lungs. Both of these are also stimulants, especially pseudoephedrine, which is apparently a key ingredient in methamphetamines, a.k.a. “meth” or “speed”. (This is why pseudoephedrine products are now regulated in the U.S. when they used to be off-the-shelf. You have to get them from a pharmacist and register and sign for them, and they can only sell you a week’s worth at a time without a prescription for a stronger dose.)

The other stuff you see in cold medicines are expectorants, which make it easier to clear out the stuff that’s gumming up your breathing (drink plenty of water with those), and cough suppressants, which also have been known to make people sleepy.

And of course it is recommended that people taking strong antihistamines or cough suppressants should not drive or operate heavy or dangerous equipment while using this stuff…