Hi waterdogs:
The advice that you got from Rick and Bob to try (any) medicine ahead of time when you will be taking it with diving is excellent advice. Any medicine that is supposed to have an effect can also have side-effects- even if it's over-the-counter (OTC) or "all natural". Rick's rapid heartbeat reaction to Sudafed is much more common than Bob's sleepiness, but even rare reactions occur in some people. You'll want to know how a medicine sets with you before you dive on it.
About "Sudafed". I have counted no less than 11 different formulations sold under the "Sudafed" name. When you discuss medicine, it's important to know exactly what you're talking about since different medicines have different effects and side-effects. For example, "Sudafed Cold and Allergy Tablets" contain pseudoephedrine (the medicine most commonly associated with the "Sudafed" name-see below) and chlorpheniramine maleate- an antihistamine that causes drowsiness in some people. I have a personal bias against combination medicines unless it is clear that you will always need all of the different medicines in the combo pill. I'd much rather just take what I need when I need it and leave out any "extras" that I could do without. I think it interesting that your physician wrote a prescription for "Sudafed" since all of the "Sudafeds" that I'm aware of are available OTC in the US without a prescription. You should find out exactly what you have.
Now, about pseudoephedrine. When most divers talk about "Sudafed" they usually mean the active ingredient that I think is common to all of the "Sudafed" formulations- pseudoephedrine. Pseudoephedrine is a sympathomimetic amine in the same general family as epinephrine (adrenaline). It can cause dizziness, insomnia, agitation/nervousness, headache, palpitations, rapid heartbeat, and other side-effects. It works by constricting blood vessels- particularly those of the upper respiratory tract- reducing swelling of the linings of the nose, sinuses, etc. Pseudoephedrine is absorbed rapidly and easily through the intestinal tract and goes to work quickly. The "half-life" of a typical non-extended release 30mg dose in most people (the time it takes for half the drug to be eliminated) is about 4 hours, but that rate of elimination can be variable between divers.
Let's talk about some of the specific formulations of the "Sudafed" brand commonly used by divers and their differences.
"Sudafed Nasal Decongestant Tablets" have 30mg of non-extended release pseudoephedrine. It takes effect quickly (I'd guess within the hour in most people- but that's just a guess) and typically continues to have the desired effect for 4-6 hours. If you take it the night before a morning dive and don't take it again before the dive, it will no longer be working and you might even have some "rebound" from the effect of the drug wearing off and be more congested than before. For most people it would probably work best to take it an hour or so before the dive, and I would personally plan to be finished diving 3-4 hours later when the effects of the drug will possibly be waning.
"Sudafed 12 Hour Tablets" contain 120mg of pseudoephedrine. It looks to me like all of the pseudoephedrine in this formulation is in the time-release form. That would suggest to me that it would take longer to take effect than the "Sudafed Nasal Decongestant Tablets", but since it is released continuously during its course of action, the time it would begin to wear off should be more predictable. "Sudafed 12 Hour Tablets" (or their generic equivalent) are my personal decongestant of choice on those occasions when I take pseudoephedrine and dive. They don't usually cause me to have insomnia, so I generally start it the night before a morning dive, and take it again in the morning the day of the dive- about 12 hours after the first dose. The effects of the drug should last my whole morning of diving and into the afternoon (or even evening).
"Sudafed 24 Hour Tablets" contain 240mg of pseudoephedrine. 60mg of the pseudoephedrine is in an immediate-release coating and 180mg is in a time-release form. It should take effect quickly when taken the morning of a dive (like the "Sudafed Nasal Decongestant Tablets" above), yet reliably last for a full day of diving with its time-release throughout the day- perhaps even including a night dive.
Again, drugs affect different people differently, and any drug can have untoward side effects. It's a good idea to try any new medicine before diving. You should be fully aware of what you're taking, the medicine's planned effects, and its potential side-effects before taking it when diving.
HTH,
Bill
The above information is intended for discussion purposes only and is not meant as specific medical advice for any individual. All questions concerning medicines and/or medical conditions and diving should be discussed between the diver and their personal physician and/or DAN before diving.
