Diphenhydramine, sold as the branded medication Benadryl, is a relatively safe medication for dogs and cats. While it can be effective in treating many conditions in dogs, it is not effective in cats. It may be helpful in treating a rash in dogs, depending upon what is causing the rash, and is commonly given to canines for its antihistamine qualities in controlling itching caused by allergies.
There are many conditions that can be causing your dog's skin rash. It is best to take your pet to a veterinarian for a definite diagnosis. However, most of these conditions may respond well to Diphenhydramine. Causes may be environmental allergies, food allergies, poison ivy/oak/sumac, an insect bite, mange, intestinal parasites, or topical parasites such as fleas, ticks or lice.
When you go to the store to buy Diphenhydramine for your dog, you'll be confronted by an array of Benadryl-like products. It is imperative that your buy a product that contains only one active ingredient and that active ingredient should be Diphenhydramine. Some formulas are including additional active ingredients to treat additional symptoms in humans but some of the additional active ingredients can be toxic in dogs. It's also imperative that you buy the capsule or pill of Diphenhydramine. Liquid solutions have an alcohol content that is too high for safe usage in animals.
The dosage of Diphenhydramine most often recommended for dogs is one to two mg of the medication per pound of body weight. For example, if your dog weighs 50 pounds, you could give your pet up to 100 mg of Diphenhydramine. It is always best to start with the lowest dosage, however, to see if it is effective. The medication can be given every eight hours. Most pet guardians find it easiest to provide the medication to their dog inside a treat, such as rolled up in peanut butter or cheese.
Until you can get your pet to a veterinarian, or until the condition resolves itself, there are additional therapies you can use in conjunction with Diphenhydramine. You might try an aloe vera cream or gel on the pet's skin. This will provide some relief from the irritation your pet is feeling. An oatmeal bath helps the skin to heal and may provide some relief for the itching that is likely accompanying the rash. A paste made from mixing baking soda and water can provide nearly instant relief in certain situations, especially in the case of an insect bite or sting.
A word of caution on aloe vera: commercially prepared aloe vera gels are fine to use on your dog, but never apply aloe straight from the aloe leaf, as this could be harmful if the dog licks it. Pure aloe contains saponins, which can be toxic if ingested by the dog.
Diphenhydramine may also reduce anxiety, reduce vertigo and cause mild sedation in pets and is sometimes given for these purposes. It can be effective in controlling vomiting, treating motion sickness and is useful in dogs who have been snake bitten, have vaccination reactions or that have been stung by bees.