You should discuss giving any over-the-counter medication with your veterinarian before giving it to your cat. This is because many human medications are toxic, some may have very different doses than you could expect and some are not used the same way. Benadryl can be used in cats safely but in general this depends on the dose. Benadryl is not often used, in part because antihistamines can have limited value in some diseases and other medications may work better. Cats do not typically have hay fever like we do and most time allergies surface as skin issues not breathing problems. Hope this information helps.
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My mother-in-law gives her cats Benadryl if they have breathing problems. Is Benadryl 
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