Quercetin is naturally in foods like berries and apples (especially with the skin on). Benadryl can help with the side effects of excess histamine sometimes associated with mast cell tumors. How Much Benadryl For A Dog - Petrefine The main side effect of giving your dog benadryl is drowsiness. How long does benadryl affect dogs . Benadryl is an antihistamine and is often given to dogs to help with allergic reactions. This side effect can be put to good use or, better said, to help dogs suffering from anxiety. What dogs should not take benadryl? It takes about 30 minutes for benadryl to start working for your pet,. How will my dog act while under the effects of benadryl? Benadryl helps with that, too. Benadryl typically takes effect within 30 minutes of administration. Diarrhea, vomiting and decreased appetite are possible, but not common. How long does benadryl last for a dog? It usually takes 24 hours for benadryl to wear off and stop working. Give your ...