Your dog was fine an hour ago, and now there are raised bumps all over the coat, or the muzzle and eyes look puffy. This is usually an allergic skin reaction called hives (urticaria), sometimes with deeper swelling called angioedema. Hives often fade on their own within a day [1], but a few signs mean an emergency, and those are the ones to know.
What hives look like
Hives are raised, short-lived bumps (wheals) that come on suddenly. They form when fluid and inflammatory cells collect in the skin. [1] They can be limited to one area or spread over the whole body, the hair over them may stand up, and they may or may not itch. [1]
Hives are easier to see on short-coated dogs, which is one reason breeds such as boxers and pit bulls seem to get them more often. [1]
Facial swelling
Angioedema is a sudden swelling deeper under the skin. It is often marked, the skin over it can look red, and it tends to affect the head and the lower legs. [1] On the face, this is the puffy muzzle, lips or eyes that owners notice. Bumps may or may not be present at the same time. [1]
What causes hives in dogs?
Hives start when mast cells in the skin release histamine and other chemicals that make small blood vessels leak. This is usually an allergic reaction, but it can also happen without allergy. [1] In dogs, triggers backed by studies or case reports include: [1]
- Insect stings and other venoms
- Vaccines
- Medicines, including anesthetics
- Foods
- Plants
- Blood transfusions and contrast dyes used for imaging
Heat, exercise, sunlight, being in heat and intestinal worms are mentioned in textbooks as rare causes, but the evidence for them is only anecdotal. [1] Dogs with atopic dermatitis appear more likely to have hives or anaphylaxis, whatever the trigger. [1] Often the exact cause is never found. [1]
After a sting
Suspected insect stings are the most often reported cause of anaphylaxis in dogs. [2][3] If your dog has had a severe reaction to a sting before, tell your vet.
After a vaccine or medicine
Vaccines and medicines are both on the list of known triggers. [1] If your dog breaks out in hives or swells up after a vaccine or a new medicine, call your vet straight away and ask whether it needs to be seen.
A study of more than 1.2 million vaccinated dogs counted about 38 adverse events, including allergic reactions, hives and anaphylaxis, per 10,000 dogs in the three days after vaccination. [4] Smaller dogs had a higher rate than bigger ones, and the risk rose with every extra vaccine given at the same visit. [4] Young adult, small-breed, neutered dogs given several vaccines at once were the most at risk. [4] If your dog has reacted before, mention it before the next vaccine visit.
How fast it starts and how long it lasts
Hives appear suddenly, and each bump usually goes away within a day. [1] New bumps can keep coming for much longer if the trigger is still around. [1] In one study of dogs with anaphylaxis after a suspected sting, most dogs reached the clinic about an hour after their signs began. [2]
When it is an emergency: anaphylaxis
Anaphylaxis is a severe allergic reaction that affects several organs at once, not just the skin. [2] In a study of 25 dogs with anaphylaxis after a suspected sting, almost nine in ten had stomach and gut signs and more than eight in ten had circulation problems, such as an abnormal heart rate or low blood pressure. [2] About one in four were breathing faster or with effort. [2] Warning signs include:
- Vomiting, retching or diarrhea [2]
- Heavy drooling [2]
- Weakness, pale gums or collapse (signs of poor circulation)
- Fast or labored breathing [2]
If you think your dog is having an anaphylactic reaction, get emergency veterinary help immediately. Fast treatment can save its life. [1]
How vets treat hives
The first step is to find and avoid the trigger where possible. [1] Vets may use anti-inflammatory medicine to calm the reaction, and anti-allergy medicine is mainly used to help prevent new episodes. [1] A severe swelling or anaphylactic reaction needs emergency medicine given by the vet. [1] Do not give your dog human allergy medicine unless your vet tells you to.
If your dog’s hives keep coming back and it is itchy, your vet may suggest a food elimination trial and then look into environmental allergies. [1] See seasonal allergies in dogs and why is my dog so itchy?.
When to see your vet
Emergency (go now):
- Vomiting, diarrhea, drooling, pale gums, weakness or collapse after a sting, injection, food or medicine [2]
- Any sign of trouble breathing [2]
Call your vet promptly:
- Hives or swelling after a vaccine or medication [1]
- Facial swelling so severe the eyes are closing
- Hives that do not settle, or keep coming back [1]
FAQ
Will my dog’s hives go away on their own?
Often, yes. Each bump usually fades within a day, but new ones can keep appearing while the trigger is still there. [1]
Why is my dog’s face swollen?
An allergic reaction can cause deep swelling (angioedema), which often shows up on the head. [1] Other problems can also cause facial swelling, so if you are unsure, call your vet.
Can I give my dog an antihistamine?
Ask your vet first. Anti-allergy medicine has a role mainly in preventing new episodes [1], and the right medicine and dose depend on your dog.
My dog got hives after vaccines. Should we skip them?
Talk to your vet. In one large study, small dogs and dogs given several vaccines at one visit had a higher risk of reactions. [4] Your vet can plan future visits with that in mind.
The bottom line
Hives and facial swelling in dogs often fade within a day. Call your vet if they follow a vaccine or medicine, and treat vomiting, drooling, pale gums, weakness, breathing trouble or collapse after a sting, injection or food as an emergency.
This article is for general information and is not a substitute for veterinary advice. If your dog has a sudden allergic reaction, contact your veterinarian or an emergency clinic.
Sources
- Torres SMF. 2.9 Urticaria: Small and Large Animals. In: Small and Large Animal Dermatology Handbook, Vol. 1. University of Minnesota Libraries Publishing, 2025. CC BY 4.0. https://open.lib.umn.edu/animaldermatology/chapter/urticaria/
- Turner K, Boyd C, Rossi G, et al. Allergy, inflammation, hepatopathy and coagulation biomarkers in dogs with suspected anaphylaxis due to insect envenomation. Front Vet Sci. 2022;9:875339. CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC9393546/
- Rostaher A, Fischer NM, Vigani A, et al. Hymenoptera Venom Immunotherapy in Dogs: Safety and Clinical Efficacy. Animals (Basel). 2023;13(19):3002. PMID 37835609. https://pmc.ncbi.nlm.nih.gov/articles/PMC10571593/
- Moore GE, Guptill LF, Ward MP, et al. Adverse events diagnosed within three days of vaccine administration in dogs. J Am Vet Med Assoc. 2005;227(7):1102-8. PMID 16220670. https://pubmed.ncbi.nlm.nih.gov/16220670/