Howdy russ:
Good question. Your line of reasoning is probably correct. I'm not aware of any studies specifically looking at phenylephrine and diving, but it is generally considered to be a relatively weak central nervous system (CNS) stimulant. Phenylephrine and pseudoephedrine (eg Sudafed) are both in the same family of decongestant drugs, but I suspect that phenylephrine has less CNS effect. Pseudoephedrine is used more commonly by divers, but that's probably because it's available over-the-counter (OTC) whereas oral phenylephrine is generally by prescription in the US.
Besides being generally (exclusively?) by prescription in the US, a problem with oral phenylephrine is that it usually comes combined with other medications in the pill. Chlorpheniramine is an antihistamine that is frequently combined with phenylephrine, and it can be sedating. Phenylpropanolamine is another decongestant in the phenylephrine family of drugs that is sometimes found in the same pill. It also probably has less CNS effect than pseudoephedrine, but when you combine it with phenylephrine, I don't know that you gain any margin of safety for nitrox diving over pseudoephedrine alone.
Phenylephrine is also available as a nasal spray OTC. It works well for short periods of time, but nasal sprays have significant "rebound" problems when used over several days and when you stop taking them. There is more info on nasal sprays elsewhere in the forum for those interested in searching for it.
FWIW, guaifenesin is an expectorant that is supposed to help "loosen" mucus and make it easier to clear. There's some debate as to whether it actually works, but I don't know of any harm in using it while diving.
For those readers who are curious about reading more concerning pseudoephedrine (eg Sudafed) and nitrox, take a look at:
http://www.scubaboard.com/showthread.php?s=&threadid=570&highlight=pseudoephedrine+AND+dan
HTH,
Bill
The above information is intended for discussion purposes only and is not meant as specific medical advice for any individual.
