We cover everything you need to know about Benadryl for dogs. How much to give, how to give, the side effects to avoid, things to know, and much more inside. Most side-effects and symptoms occur within the first 1 to 5 hours after Benadryl exposure. Therefore it is highly advisable to have your dog closely monitored during this timeline. The side-effects and symptoms Benadryl may cause are divided in two major groups – common and rare. Learn about the potential side effects of Benadryl (diphenhydramine). Includes common and rare side effects information for consumers and healthcare professionals. Includes common and rare side effects information for consumers and healthcare professionals. The side effects of Benadryl in dogs are similar to the side effects humans might experience with the drug. These include sleepiness, dry mouth, sluggishness, or urinary retention. What are the side effects of Benadryl? Many dog owners use Benadryl for controlling the symptoms of allergies and motion sickness in their pets with no problems whatsoever, but although Benadryl is considered relatively safe for use in dogs and other pets, it can produce unwanted side effects on occasion, so it is always a good idea to consult. Benadryl is an over-the-counter medicine that can be given to dogs. Diphenhydramine, the most common active ingredient, is an H 1 antagonist, which means it helps to prevent the uptake of histamine (a type of substance responsible for allergic reactions) by receptors in the body. The effects of Benadryl will usually begin 30 minutes after your dog has taken it. When using this medicine to prevent motion sickness, you should give your dog this drug 20 minutes before the journey begins. One of the side effects of giving your dog Benadryl is drowsiness. This is because of the active ingredient of diphenhydramine and can be used for dogs that suffer from anxiety as it helps to calm them. FINDING THE CORRECT BENADRYL DOSAGE FOR DOGS. The usual dose of Benadryl for dogs (adult) is 2 to 4 milligrams per kilogram of body weight, which corresponds to a dose of 0.9 to 1.8 miligrams per pound of body weight, according to the Merck Veterinary Manual. Are you considering giving your dog Benadryl to treat allergies, anxiety, or motion sickness? Learn more about the side effects of Benadryl for dogs here!.