Although most OTC and scrip medications for allergies don't have an affect on carb consumption, it's quite possible they affect affect metabolism and very likely they will affect absorbtion/loss of vitamins and minerals and affect your hydration levels. If you can't stand it, the temporary relief from anti hists may be worth risking a possible temporary plateau.

Another possibility that should have no effect on LCing at all is called "Nasal Lavage". It is extremely useful in moderating sinusitis and sinus infection, but I find it also reduces allergies symptoms dramatically. There are a number of different methods and devices, but I find the best is the Grossan nasal irrigator, a special tip that snaps onto a Water Pik. You can get info at http://www.pharmacy-solutions.com/. They have a variety of other devices to accomplish the same thing. It takes a little getting used to, but well worth doing a few times a week to prevent symptoms.