The Pfizer company received FDA approval for an over-the-counter ("OTC") version of its anti-allergy medication, Zyrtec over a year ago. There was the usual appropriate amount of hype about how wonderful it is.
It IS a good antihistamine however it does produce a significant amount of drowsiness in some persons and therefore is not FAA approved for use while flying.
Claritin and Allegra are also good anti-allergy medicines and do not produce drowsiness, (i.e., are FAA approved). Allegra remains prescription only and Claritin is available over the counter. Claritin works pretty well. If it does not seem to help after several days of use, try TWO tablets instead of the label directions. It is available at SAM's, Costco's, etc. as "loratadine" at a very reasonable price (less than $13.00 for 300 tablets).
If you do take Zyrtec, allow 36 hours to elapse from the last dose before resuming crew duty. updated 3/28/2010