Treatment
Emergency treatment of anaphylactic shock involves asministering intravenous or subcutaneous (under the skin) adrenaline, oxygen, antihistamines, IV fluids, and hydrocortisone - which are not available in the home. This is why it is best to have your veterinarian give vaccines - he or she has the drugs and equipment to treat allergic reactions in time.
If you see symptoms that appear to be anaphylactic shock, give your pet an antihistamine like Benadryl as soon as possible. Pets suffering from anaphlyaxis may have trouble swallowing because their throats often swell, making the pills difficult to administer. The liquid form of Benadryl usually comes in a dose of 12.5mg per teaspoon and pills are usually 25mg each. Pets will need 1mg per pound of body weight every 6 - 8 hours. This can help bring down the swelling and ease his breathing, but this is a life threatening emergency and you must take your pet to the vet right away.
Prevention
A dog who has had an allergic reaction to a drug in the past should not be given that drug again