How to Know if your Pet Is Having a Vaccine Reaction 

There is always a possibility that your pet could have a vaccine reaction. Reactions vary in type and degree. A “reaction” is a symptom or something that is abnormal to your pet’s regular health condition or in the way it is acting. A pet may act differently if it is in pain. Pain is a reaction, but not necessarily an allergic reaction. Depending on the severity, treatment for the pain may be necessary.  

Vaccine reactions can increase in severity with age and/or with the accumulation of vaccines. Reactions can start out “mild”, but rapidly increase to a “severe” level which could ultimately result in death. Symptoms may include swelling, fever, vomiting, diarrhea, and/or other abnormal conditions. 

If your pet shows any sign of a reaction to a vaccine immediately following the injection, always notify the attending veterinarian immediately. If your pet shows a sign of illness or lethargy in the 48 hours following vaccination, contact your regular veterinarian or any licensed veterinarian or an emergency pet hospital. 

It is not unusual for a pet to be quiet and uncomfortable for up to 24 hours after a vaccination. There may be localized or generalized pain, lethargy (diminished activity or decrease in social behavior), loss of appetite, mild swelling at the injection site, or even a low/mild fever associated with the shots. (Normal temperature for dogs and cats usually range from 100.5 to 102.5 degrees). If any of these signs persist longer than 24 hours, bring your pet to a licensed veterinarian for an examination. 

In rare occasions, more severe side effects may occur within minutes or hours of the vaccination. These are allergic reactions called “ANAPHYLACTIC” reactions. Symptoms of an anaphylactic reaction may include any of the following (or possibly other) symptoms: Swelling to face and legs, whole or partial body itching, hives, nausea, vomiting, diarrhea, difficulty breathing, choking, seizures, staggering, and/or collapse. These may appear almost immediately after the vaccinations are administered, or possibly hours after. Any of these signs may start out in a mild form, but can progress rapidly to a more severe anaphylactic reaction which could result in death. IF YOU NOTICE ANY OF THESE REACTIONS, IMMEDIATELY BRING YOUR PET TO A LICENSED VETERINARIAN OR TO THE CLOSEST EMERGENCY PET HOSPITAL. EARLY TREATMENT OF THESE SIGNS MAY PREVENT MORE SIGNIFICANT PROBLEMS FROM DEVELOPING AND SAVE YOUR PET’S LIFE. 

IF THERE IS ANY QUESTION WHETHER YOUR PET IS HAVING AN ALLERGIC REACTION, IMMEDIATELY CONTACT YOUR VETERINARIAN OR ANY LICENSED VETERINARIAN OR AN EMERGENCY PET CARE FACILITY. 

Post-Vaccine Care 

IF YOUR PET IS EXPERIENCING MILD PAIN OR DISCOMFORT OR BECOMES INACTIVE AFTER RECEIVING A VACCINE, you may apply a WARM compress (NOT HOT) to the vaccination site every 6 to 8 hours within the first 24 hours after vaccination for approximately 15 minutes per application. You may also give your pet Benadryl (Diphenhydramine) as explained below.  Contact a licensed veterinarian if your pet appears to be in significant pain.  

NEVER give any of the following to a pet, as it could cause severe illness or death:  

~ Tylenol  

~ Plain Aspirin  

 ~ Flavored medications  

 ~ Diphenhydramine to a pet weighing under 3 pounds 

~ Diphendydramine or any medication if the pet is currently taking other medication or is under the care  of a veterinarian for a specific health condition. Contact your veterinarian to seek further advice.

With the exceptions stated above, you may immediately give your pet Benadryl (Diphenhydramine), tablets or liquid form every 12 hours for 2 to 3 days. The dose is approximately one-half of your pet’s weight. For example, if your pet weighs 10 pounds, you would administer 5 mg of Diphendydramine every 12 hours for two to three days. 

IMPORTANT: Weigh your pet prior to administering any medication to get an accurate weight for proper dosing! 

If your pet becomes listless or sleepy after Diphenhydramine is given, you may cut the dose in half or completely stop the medication. 

Diphenhydramine Dosing for Dogs & Cats = 2-4 mg/kg. 

Dosing Examples: 

Pet’s Weight Dose (25 mg tablet) Dose (12.5 mg/5ml liquid) 

12 lbs 1/2 tablet – or – 5 ml liquid  

25 lbs 1 tablet – or – 10 ml liquid  

50 lbs 2 tablets – or – 20 ml liquid