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dye-fen-HYE-drah-meen - Pronunciation guide

Brand Names

  • Benadryl
  • Vetadryl™ 30 Tablets


Rx symbolDiphenhydramine is an antihistamine that inhibits histamine binding at the H1 receptors.  Antihistamines do not block the release of histamine but compete for uptake at the receptors on sensitive cells in the respiratory tract, intestines, blood vessels, and skin.


Diphenhydramine is used to treat allergies, insect bites and stings and other causes of itching. It is also used for its sedative and antiemetic effects in treatment of travel anxiety, and for its antitussive effect in cases of coughs.

Dosage and Administration

Prescription medicationDiphenhydramine
Method Dosage
(click row for calculator)
Concentration Period Duration
Oral 2-4 mg/kg 25 mg/tablet Twice daily NA
Oral 2-4 mg/kg 30 mg/tablet Twice daily NA
Oral 2-4 mg/kg 50 mg/tablet Twice daily NA
Intravenous or Intramuscular injection 0.5-1 mg/kg 50 mg/ml Twice daily NA


  • Federal law restricts this drug to use by or on the order of a licensed veterinarian.
  • Extra-label use of drugs in treating animals is allowable only by licensed veterinarians within the context of a valid veterinarian-client-patient relationship, and does not include drug use in treating animals by the layman (except under the supervision of a licensed veterinarian).
  • Calculator is for educational purposes only. Follow your veterinarian's instructions regarding use of this, or any medication.
Method Amount Dosage Period Duration Note

60 adult

10-20 foal

ml Daily Consult vet

For cough.

Benylin syrup for Cough

IV, IM 0.25 mg/kg Daily Consult vet

For hives

Dose form; 10mg/ml, 14.5mg/ml and 50 mg/ml

Oral 0.35-0.70 mg/kg Up to 4 times daily Consult vet

Dose form: 25 and 50 mg capsules

Use of this drug for horses requires a veterinary order.

Side Effects

The most common side effects of diphenhydramine are sedation and drowsiness. Other side effects include dry mouth, urinary retention, and rarely, diarrhea, excitement, tremors and seizures, and loss of appetite.


Individual horses may react differently to diphenhydramine and dosage should be based on the horse's age and condition. Diphenhydramine may thicken mucus in the respiratory tract, so extra precautions should be taken when used to treat horses with respiratory problems.

Directions for administration should be followed and it should not be used in animals that are allergic to it and other antihistamines.  Do not use in animals with glaucoma, that are pregnant or nursing and use caution when given with other CNS-depressant drugs such as tranquilizers or barbiturates.

Diphenhydramine is not FDA approved for use in horses, but it is commonly used and considered acceptable practice. Although over-the counter forms are available, this drug should be used by or upon a written or oral order of a licensed veterinarian.

Diphenhydramine is prohibited or regulated in most sanctioned competitions. Consulting the individual regulatory group is essential for determining provisions in rules for the therapeutic use of these substances.


Diphenhydramine may affect the activity of anticoagulant drugs such as warfarin. When combined with other central nervous system depressant drugs, such as tranquilizers, antihistamines have an additive effect. Diphenhydramine may enhance the effects of epinephrine.


Overdoses may cause increased sedation and severity of other side effects and can cause symptoms ranging from CNS stimulation to CNS depression. Clumsiness, seizures, respiratory depression, coma and death can occur after a massive overdose..


Benadryl Benadryl

Vetadryl Tablets Vetadryl Tablets



About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.