Over the counter diphenhydramine (Benadryl) or comparable antihistamines can be used in an emergency for horses with severe hypersensitivity or allergy. Prolonged use is not advised. There is no literature to support the use of diphenhydramine in horses. You are using this product at your own risk.
If possible, you should always contact your vet before you use any emergency medications. If the diagnosis is wrong or the medication affects the horse adversely, you could endanger your horse’s life.
Assuming you cannot contact your vet: For hives or possible allergic airway disease (heaves), for a 1000 lb horse, give 5-10 tabs of 25mg diphenydramine crushed up and mixed in water and dosed in a syringe orally (by mouth). If signs of the problem have not abated after 60 minutes, and you still have no access to a veterinarian, give an additional 5 tabs. It may be wise to keep 20 tabs on hand in your emergency kit.
Diphenydramine can cause side effects of excitability in horses. It should only be used under the guidance of your veterinarian.