If your cat is experiencing a severe allergic reaction, you should take them to the veterinary clinic for an emergency appointment. If they have mild symptoms that you think might be caused by an allergy, they may be able to have Benadryl. Benadryl is safe for most cats, but there are some situations where Benadryl might not be the best choice to get your kitty on the mend.
As long as you speak to your veterinarian first and check the label carefully, Benadryl might be an excellent option to give your cat some relief.