Howdy Cory:
Zyrtec D contains 5mg of the antihistamine cetirizine and 120mg of the decongestant pseudoephedrine. The recommended dosing is one every 12 hours.
Zyrtec is being marketed as a "less-drowsy" antihistamine, but it can still cause drowsiness in up to 14% of users. It can also cause a dry mouth and fatigue. A diver should be aware of these possibilities. Since it is an antihistamine, it might be unwise to mix it with other antihistamines. Common seasickness medicines like Dramamine, Bonine, Triptone, etc. are antihistamines. I'm not aware of any studies specifically looking at the safety of Zyrtec while diving, but there are no obvious reasons not to take it if there are no significant untoward reactions to the medicine.
The pseudoephedrine in Zyrtec D is essentially the same as in a Sudafed 12 hour tablet. At least one study has found pseudoephedrine safe for divers, but there is a theoretical concern about it's use in diving. It is a central nervous system stimulant and might increase a diver's sensitivity to oxygen toxicity. DAN published an article in their Nov/Dec '99 Alert Diver addressing this concern. Their conclusion was that occasional use of pseudoephedrine while diving air is unlikely to cause a problem. They also seemed to have no significant problem with mixing occasional pseudephedrine with oxygen partial pressures of 1.4ATA or less like you might see when diving nitrox conservatively. They did recommend against daily use of pseudoephedrine when diving air or nitrox, or any use of pseudoephedrine with PO2's greater than 1.4ATA. You can read more about pseudoephedrine at:
http://www.scubaboard.com/showthread.php?s=&threadid=3191&highlight=nitrox
As with any medicine taken for sinus congestion, the diver needs to be aware of the possibility of barotrauma from a "squeeze" or even a "reverse squeeze" if the medicine should be ineffective at depth. If a diver MUST take a decongestant in order to dive, they might want to consider aborting the dive. Medications should also be timed so they won't "wear off" while diving.
HTH,
Bill
The above information is provided for discussion purposes only and is not meant as specific medical advice for any individual.
