Cats can have Benadryl (diphenhydramine) safely, but only under veterinary guidance to ensure the proper dosage and to check for any contraindications. It is commonly used for allergic reactions in cats, such as from bee stings or vaccine reactions, and sometimes for mild sedation. However, not all cats respond the same, and Benadryl can cause side effects like sedation, dry eyes, loss of appetite, or agitation in some cases. It is crucial to avoid giving Benadryl formulations combined with other medications like pain relievers or decongestants, and cats with certain health conditions should not receive it without vet approval. Typical dosing is about 1 mg per pound of the cat's body weight, given every 8-12 hours if needed, but only after consulting a vet for personalized advice.