Claritin (loratadin generic) is my friend this time of year. I go from
feeling I have a cold coming on to normal in an hour or so. No
sleepiness like Benadryl causes.

In NZ, my Aviation Medical Assessor told me that the rules for my PPL
medical certificate meant that all oral antihistamines were
unacceptable and I should ground myself for 24hrs after using them.
When I asked about the "non drowsiness" type like Claratyne
(Loratadin) he said that it was included and for those one-pill-a-day
types I should ground myself for even longer. Nasal sprays like
Beconase / Alanase / Beclomethasone are acceptable.

Medical certificate rules for Gliding in NZ are different than for PPL
but still preclude the use of antihistamines.

YMMV
Neil