Greg, you're right about the "speed" aspect of medications like Sudafed. As memory serves, a Canadian sculler was sanctioned at one of the Summer Olympic events because she inadvertedly used the wrong over the counter (OTC) decongestant which, as it happens, contained pseudoephedrine. And of course even though the medication is OTC in many countries, it is a "banned" substance as far as the Olympics are concerned.
As I mentioned, almost all medications have counterindications or side effects. And one of those associated with the aforementioned is how it can make you "jittery" or "anxious". In fact, my pharmacist friend recommends that people who take this medication only do so BEFORE noon; if it is taken any later in the day it can contribute to sleeplessness. (Been there, done that... she's RIGHT!)
BTW, I also wanted to point out that there is a difference between a decongestant and an antihistamine. In addition to combatting allergic responses to substances, and "drying things out", antihistamines may also make some people drowsy. So medications that contain BOTH decongestants AND antihistamines are generally not recommended for people who are using them to help them dive.
Again, I'm not a physician or a pharmacist. Just a (hopefully) informed health-conscious consumer.
TTFN,
~SubMariner~