Updated October 13, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
Quick answer: Cancer symptoms in dogs depend on where the cancer is. Skin tumors show up as lumps. Lymphoma often causes swollen lymph nodes. Bone cancer causes lameness and swelling. Internal tumors may cause weight loss, weakness, a swollen belly, or sudden collapse from bleeding. Because many signs overlap with other diseases, testing is needed to know for sure.
Common cancers and their signs
- Mast cell tumors: skin lumps that may change size or get red and itchy. See mast cell tumors.
- Lymphoma: firm, painless, swollen lymph nodes, sometimes with lethargy, weight loss, or increased thirst.
- Hemangiosarcoma: often hidden until it bleeds, causing sudden weakness, pale gums, and a swollen belly. See spleen tumors.
- Osteosarcoma (bone cancer): lameness and a firm, painful swelling on a leg, especially in large and giant breeds.
- Oral melanoma and other mouth tumors: bad breath, drooling, bleeding, trouble eating, or a visible mass.
- Mammary tumors: lumps along the belly in unspayed or late-spayed females.
- Bladder cancer (urothelial carcinoma): straining, frequent urination, and blood in the urine.
- Nasal tumors: one-sided nasal discharge, nosebleeds, or facial swelling.
General signs
- Weight loss, poor appetite, lethargy
- Vomiting or diarrhea that doesn’t resolve
- Coughing or trouble breathing
When to see a vet
Book a visit for any new lump or lasting change. Go to an emergency vet for collapse, pale gums, a swollen belly, or trouble breathing.
More: early signs of cancer.
Sources
- Merck Veterinary Manual: Neoplasia in dogs
- Biller B, et al. 2016 AAHA Oncology Guidelines for Dogs and Cats. J Am Anim Hosp Assoc. 2016
- Veterinary Cancer Society: Warning signs of cancer in pets
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.