Updated October 13, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
Quick answer: Vaccines aren’t toxic, and mild reactions like soreness, tiredness, or a low fever for a day or two are normal. Rarely, dogs have allergic reactions within minutes to hours, with vomiting, facial swelling, hives, or collapse, which need emergency care. If your dog develops new signs days after a vaccine, like tremors or vomiting, consider other causes, including something it may have eaten.
Normal after vaccines
- Sleepiness
- Mild soreness or a small lump at the injection site
- Low appetite for a day
- Sneezing after intranasal vaccines
Allergic reaction (emergency)
- Vomiting or diarrhea soon after
- Facial or muzzle swelling, hives
- Trouble breathing
- Weakness, pale gums, collapse
Signs that point elsewhere
- Tremors, seizures, or agitation
- Bleeding or bruising
- Signs starting several days later
These are more likely from toxins or illness than vaccines. Check for chewed packages or plants, and call your vet or a poison hotline: ASPCA Animal Poison Control Center (888) 426-4435 or Pet Poison Helpline (855) 764-7661 (fees apply).
Sources
- Ellis JA, et al. 2022 AAHA Canine Vaccination Guidelines. J Am Anim Hosp Assoc. 2022
- Moore GE, et al. Adverse events diagnosed within three days of vaccine administration in dogs. JAVMA. 2005
- ASPCA Animal Poison Control Center: Top pet toxins
This article is for general education and is not a substitute for care from your own veterinarian. Learn about The Pet Vet and our editorial policy.