My Vet derm doesn't hear much success w/ benadryl, but thinks it's worth a try. They also say any anti-hist should be given a 10-14 day trial before evaluating its effect.
I actually like Claritin best (also 24 hr med), but so many dogs do great on Zyrtec. Another med that derms recommend is chlor-trimeton (sp?)...but that one has a short half-life similar to benadryl, so I think you have to give it 4x day like the benadryl.
I've never had any success w/ benadryl; I use it as a pre-med for vaccinations though
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