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Vaccine reaction in dogs and cats

Mild post-vaccination lethargy and a small lump are normal. Facial swelling, hives, or vomiting within 1–4 hours = call your vet. Collapse or breathing difficulty within 30 minutes = emergency vet NOW.

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In the next 60 seconds

If your pet is having a routine vaccination, wait at the practice for at least 15–30 minutes post-injection — this is when anaphylaxis is most likely to occur and the team can respond immediately.
For mild normal reactions: provide a quiet, comfortable place to rest. Offer water and gentle encouragement to eat. Monitor for the next 24–48 hours.
For moderate reactions (facial swelling, hives, vomiting): telephone your vet practice immediately and describe the signs. Do not wait and see — antihistamines and/or corticosteroids may be needed promptly.
For anaphylaxis (collapse, breathing difficulty, white gums): go to the nearest emergency vet immediately — do not wait for your own practice to open. Call ahead if possible so they can prepare adrenaline (epinephrine).
For a lump at the injection site in a cat: monitor the size and duration. Any lump present after 4 weeks, over 2 cm, or growing = telephone your vet for an urgent assessment appointment.
Do not give any human antihistamines without speaking to a vet first — some (e.g. those containing xylitol or decongestants) are toxic to pets.

Recognise the signs

NORMAL (does not require veterinary contact): mild lethargy or quietness for 24–48 hours; reduced appetite; small, firm lump at the injection site (should resolve within 2–4 weeks); mild local soreness
MODERATE (call your vet): facial swelling, particularly around the muzzle, eyes, or ears; hives (raised bumps under fur); vomiting or diarrhoea within 1–4 hours post-vaccination; persistent lethargy beyond 48 hours
SEVERE — ANAPHYLAXIS (emergency vet within minutes): collapse or extreme weakness; pale, white, or grey gums; breathing difficulty or wheezing; extreme distress; cold extremities; vomiting with collapse; typically occurs within 15–30 minutes of vaccination
DELAYED CONCERN — injection site sarcoma (cats): any lump at a vaccination site that persists beyond 4 weeks, is larger than 2 cm, or continues to grow = urgent veterinary assessment

First aid steps

If your pet is having a routine vaccination, wait at the practice for at least 15–30 minutes post-injection — this is when anaphylaxis is most likely to occur and the team can respond immediately.
For mild normal reactions: provide a quiet, comfortable place to rest. Offer water and gentle encouragement to eat. Monitor for the next 24–48 hours.
For moderate reactions (facial swelling, hives, vomiting): telephone your vet practice immediately and describe the signs. Do not wait and see — antihistamines and/or corticosteroids may be needed promptly.
For anaphylaxis (collapse, breathing difficulty, white gums): go to the nearest emergency vet immediately — do not wait for your own practice to open. Call ahead if possible so they can prepare adrenaline (epinephrine).
For a lump at the injection site in a cat: monitor the size and duration. Any lump present after 4 weeks, over 2 cm, or growing = telephone your vet for an urgent assessment appointment.
Do not give any human antihistamines without speaking to a vet first — some (e.g. those containing xylitol or decongestants) are toxic to pets.

Do NOT

Do NOT stop vaccinating your pet because of normal mild reactions — the risk of preventable disease is far greater than the risk of vaccine reactions
Do NOT administer human antihistamines without veterinary advice
Do NOT ignore a growing or persistent lump at a vaccination site in a cat
Do NOT dismiss facial swelling or vomiting post-vaccination as minor — these require prompt veterinary contact

While transporting to the vet

When to phone the vet immediately

Anaphylaxis signs within 30 minutes of vaccination
Breathing difficulty at any point post-vaccination
Collapse or loss of consciousness
Persistent lump in a cat beyond 4 weeks post-vaccination
Rapidly progressive facial swelling

When it can usually wait

Common causes

Hypersensitivity (allergic) reaction to vaccine components — adjuvants, proteins, or stabilisers
Anaphylaxis — a severe, rapid immune response (rare but life-threatening)
Normal immune activation — causing mild, transient side effects
Injection site sarcoma in cats — a rare but serious complication associated with certain vaccines (especially in the interscapular region)

What you'll need

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Pet vaccination record book

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Pet first aid kit

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What the vet will need to know

Which vaccine was given and when
Exact time post-vaccination that symptoms began
Specific symptoms observed
Current gum colour
Whether the pet is conscious and responsive
Any known previous vaccine reactions
Current medications

Aftercare

Prevention

Always stay at the practice for at least 15–30 minutes post-vaccination
Inform your vet of any previous reactions before vaccination
Keep your pet's vaccination records accurate so boosters are given at appropriate intervals — over-vaccination increases risk
For cats: request vaccination in the lower limb rather than between the shoulder blades — if a sarcoma develops in the lower limb, it is more surgically accessible
Keep a record of which vaccine brand was used in case of reaction — for future reference

Breed-specific notes

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Frequently asked questions

Is it normal for my dog to be sleepy after a vaccination?

Yes. Mild lethargy for 24–48 hours is a normal immune response to vaccination and does not require treatment. Offer rest, water, and gentle encouragement to eat. If lethargy persists beyond 48 hours, or is accompanied by any other symptoms, contact your vet.

There's a lump where my cat was vaccinated. Should I be worried?

A small, firm lump at the injection site is normal and usually resolves within 2–4 weeks. However, any lump that persists beyond 4 weeks, is larger than 2 cm in diameter, or continues to grow should be assessed urgently by a vet — it may need a biopsy to rule out injection site sarcoma.

Can I give my pet an antihistamine to help with a vaccine reaction?

Do not give any antihistamines without speaking to your vet first. Some human antihistamine products contain xylitol, decongestants, or other ingredients toxic to pets. Your vet can advise on the appropriate product and dose. For anaphylaxis, antihistamines alone are not sufficient — adrenaline is required.

Should I stop vaccinating my pet after a reaction?

Not necessarily. This is a decision to make with your vet, weighing the risk of reaction against the risk of preventable disease. Options include pre-treatment before future vaccines, choosing a different vaccine product, titre testing to assess existing immunity, or in some cases altering the vaccination schedule.

How common is anaphylaxis after pet vaccination?

Anaphylaxis after vaccination is rare — estimated at fewer than 1 in 10,000 vaccine doses in dogs, and similarly rare in cats. However, when it occurs it is life-threatening. This is why staying at the clinic for 15–30 minutes post-vaccination is important, and why vets always have emergency medications available when vaccinating.

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