Major Uses

Diphenhydramine is a commonly used antihistamine that can be administered to dogs, cats, and horses. The drug has several uses, but it is primarily used to treat allergy symptoms, itchy skin, and allergic reactions brought on by other drugs, vaccines, or insect bites. The drug's sedative properties can also be beneficial as a mild tranquilizer. Diphenhydramine also has anti-nausea effects, so it can be useful in the treatment of motion sickness and also helpful during chemotherapy to prevent vomiting.

Common Precautions

Diphenhydramine is typically a safe drug when the directions of the prescribing veterinarian are closely followed, however, side effects can occur. The most common side effect is lethargy and sedation, but in some cases this is a desired effect. Over time, some animals may become tolerant to this effect as well. Dry mouth and urinary retention are other common side effects, and some animals may experience diarrhea or a decrease in appetite. Cats may become overexcited with the use of diphenhydramine, which also can be very distasteful to them. Diphenhydramine should be used with caution in animals that have glaucoma, problems urinating, prostate enlargement, an overactive thyroid, high blood pressure, or heart disease. If any of these conditions are present, the prescribing veterinarian should be aware, as alternate medication may be necessary.

Possible drug interactions can occur, so the prescribing veterinarian should be informed of all other medications the pet is taking prior to administration of diphenhydramine. Anticholinergic drugs and CNS depressant drugs may interact with diphenhydramine.

Diphenhydramine should not be administered to animals with a known allergy or hypersensitivity to the drug. Signs of an overdose may be seizures, depression, or respiratory depression. If any of these symptoms occur, the pet's veterinarian or emergency care should be sought immediately.


Diphenhydramine should be stored in tight containers at room temperature. Proper care should be taken so that solutions do not freeze.


Diphenhydramine is available in 12.5 mg, 25 mg, and 50 mg tablets, as well as a 12.5 mg/5 mL suspension. The injectable version is available in 10 mg/mL and 50 mg/mL concentrations. The typical dosages for dogs are 0.5 - 2 mg/lb (1 - 4 mg/kg) by mouth two to three times a day or 0.5 - 1 mg/lb (1 - 2 mg/kg) intramuscularly, intravenously, or subcutaneously twice a day. For cats, the typical dose is 0.25 - 0.5 mg/lb (0.5 - 1 mg/kg) by mouth every 12 hours, but because the taste is displeasing to cats, an intramuscular injection can be chosen. The typical dose is 0.5 - 1 mg/lb (1 - 2 mg/kg) every 12 hours. For horses the typical dose is 0.25 - 0.5 mg/lb (0.5 - 1 mg/kg). Doses may vary in different species, when the drug is given by a different route or concurrently with other medications, and with regards to a patient's age, breed, and health status. A veterinarian's dosing instructions and/or those printed on the medication label should be followed closely.

If a dose is missed, it should be given as soon as possible. However, if it is almost time for the next dose, then the missed dose should be skipped and the regular dosing schedule should continue. Two doses should not be given at once.

This information is for general reference only and should not be used for the diagnosis or treatment of any condition of your pet. It's intended as a general reference, this information may not include all possible uses, precautions, directions, reactions (including allergic), drug interactions, or withdrawal times. Always consult your local veterinarian and have your pet examined for any advice concerning the diagnosis and treatment of your pet, including which products and doses are most appropriate. Any trademarks are the property of their respective owners. VetDepot is not a pharmacy. All prescription products are dispensed by our Pharmacy Partner. Article last updated 2/2014.