As with any drug, using it regularly over. However, for dogs with liver or kidney disease, the effects can last longer. Can I Give My Dog Benadryl To Calm Him Down - Rehome By Adopt-a-petcom How long does benadryl take to work in dogs? How long does it take for benadryl to calm a dog . How long does it take for benadryl to work on dogs? This post was updated on 7/12/2020 giving benadryl to dogs: Benadryl typically takes 30 minutes to start working, so plan accordingly if you plan on using it to treat anxiety or mild motion sickness. Vets often prescribe benadryl for dogs, but it's usually for allergies or itching. It usually takes 24 hours for benadryl to wear off and stop working. The effects of benadryl will usually begin 30 minutes after your dog has taken it. A double dose (2 mg/pound body weight) may be given as needed, such as in snakebites. The exact benadryl dose for your dog depends on its size. The recommended dose for benadryl in dogs is 1 mg of ben...