Diphenhydramine is a first-generation antihistamine that is frequently used in the brand-name antihistamine known as Benadryl. Benadryl formulated with just diphenhydramine is a human medicine that can used as an extra-label medication for dogs, as well. It is critical that you check the ingredients on the label as some varieties have additional ingredients such as phenylephrine, which can be dangerous to your pet. It is also important to contact your veterinarian to ensure the proper dosage for your pet, as this can vary somewhat depending on your dog’s health history, size, and other medications.

Benadryl is a well-known brand name utilizing diphenhydramine, one of the earliest antihistamines, and it can be prescribed to dogs to relieve the signs of allergies.

Although Benadryl is generally safe for dogs, some side effects can occur. Side effects usually occur within an hour of administration. They are typically mild and can include:

Overdoses of Benadryl are more often caused by your animal getting a box of the medication from the garbage or medicine cabinet than by being given too much. If you see these signs of an overdose, contact your veterinarian immediately.

Along with a version of diphenhydramine by the name of Vetadryl developed specifically for cats and dogs, there are a few other first generation antihistamines that can be used for dogs. These can include:

Chlorpheniramine - This antihistamine is frequently used in small animal veterinary practice and is less likely to cause drowsiness than some of the others

Clemastine - Clemastine is classified as an antipruritic (anti-itch), as well as a sedating antihistamine, and is tends to exhibit fewer side effects

Hydroxyzine - This drug is a more aggressive antihistamine, but also has more potent side effects 

Meclizine - Commonly used to reduce nausea, particularly nausea caused by motion sickness

Allergies in dogs are the defensive mechanism of the immune system which employs specialized immune cells in the dog’s body to defend it against a substance that it sees as an invader. The specialized cells the body uses to protect itself are known as mast cells, and when the immune system is stimulated by allergens, the mast cells release histamine into animal’s system, which has an inflammatory effect on any tissues that it contacts. This results in the itchy and inflamed skin conditions characteristic of most allergic reactions in dogs. Benadryl, when acting as a remedy for allergies, like other first generation antihistamines, blocks the receptors that the histamine would attach to, thereby stopping the reaction to the histamine. However rare, an allergic reaction to the Benadryl itself is possible.

Toxicity resulting from a large ingestion of a product like Benadryl is caused by the overconsumption of ingredients in the antihistamine. Safe under normal circumstances, the overdose of products that cause effects like drowsiness, muscle tremors, and agitation can be dangerous for your pet, leading to poisoning.

Unless the consumption of the medication was witnessed, the diagnosis of allergic reaction or poisoning is likely to be driven by the signs, and supportive treatments will be started as soon as possible. If you see your dog  ingest a large amount of Benadryl, or he seems to be having a reaction to prescribed Benadryl, gather as much information about the incident as possible and provide the details to the veterinary team upon arrival at the clinic. The age and weight of your dog will be measured, and a urinalysis, complete blood count, and biochemistry profile will also likely be recommended at the veterinarian's office at this time as well. 

Currently, there are no reliable tests that can detect the substance in the blood or urine, so the diagnosis is based on clinical signs and timing. It is helpful to have the packaging on hand. The packaging can help determine the strength of the particular type of Benadryl that your pet consumed, as well as ensuring that toxicity from any additional ingredients is addressed before it becomes a problem.

If you see your dog consume large quantities of Benadryl, but they remain asymptomatic, your veterinarian may instruct you on how to properly induce vomiting. When you reach the veterinarian’s office the treatment of an overdose of Benadryl, or other antihistamines, is mainly supportive. Activated charcoal may be administered to your pet as well in order to soak up as much of the toxin in the stomach as possible and in some cases, a gastric lavage to empty the patient’s stomach, will be initiated. 

IV fluids will most likely be given to prevent dehydration or imbalances in electrolytes, and medications may be given to ease both the tremors and the increased heart rate. It is of particular importance that the temperature is closely monitored so that fevers may be addressed. As overdoses of antihistamines are known to cause a rapid heart rate and increased blood pressure, monitoring the functioning of the cardiovascular system is also critical.

This medication should not be given to dogs with certain disorders as they may exacerbate them. If your pet has any of these disorders, it is important to let the treating veterinarian know before administering any sort of antihistamine. These disorders may include: 

Benadryl is also known to react negatively with several types of medication. Alert your pet’s doctor if your dog is taking any sort of other drugs. Medications such as tranquilizers, heparin, epinephrine, or barbiturates are known to interact with diphenhydramine, and can be dangerous when taken in conjunction with Benadryl.