Updated September 25, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
Quick answer: Dogs with cancer, and dogs on chemotherapy, can develop emergencies that need immediate care. Go to an emergency vet for collapse, pale gums, a swelling belly, trouble breathing, seizures, fever during chemotherapy, repeated vomiting, inability to urinate, or sudden severe lameness. Fast treatment can be life-saving, and even when a cancer isn’t curable, emergency care can relieve suffering.
Emergency signs
- Collapse, weakness, or pale gums: possible internal bleeding, common with hemangiosarcoma
- A rapidly swelling belly
- Difficulty breathing: fluid around the lungs or lung involvement
- Seizures
- Fever, lethargy, or not eating during chemotherapy: may mean a dangerously low white blood cell count and infection
- Repeated vomiting or bloody diarrhea
- Straining without producing urine
- Sudden severe lameness in a dog with bone cancer (possible fracture)
- Uncontrolled pain
- Excessive thirst, weakness, and vomiting from high calcium, which some cancers cause
What to do
- Call ahead to the emergency clinic.
- Bring a list of medications and your dog’s treatment records.
- Tell the team if your dog is on chemotherapy and when the last dose was given.
Being prepared
- Keep your oncologist’s after-hours instructions handy.
- Take your dog’s temperature if advised, and know what reading to call about.
- Know the nearest 24-hour emergency hospital.
More: living with cancer long term.
Sources
- Biller B, et al. 2016 AAHA Oncology Guidelines for Dogs and Cats. J Am Anim Hosp Assoc. 2016
- Merck Veterinary Manual: Oncologic emergencies in small animals
- Merck Veterinary Manual: Hemangiosarcoma in dogs
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.