8 Warning Signs of Cancer in Dogs That Are Easy to Miss — And What to Do About Each One

Cancer is the leading cause of death in dogs over 10, affecting nearly 50% who reach that milestone. Yet many early warning signs are subtle enough to be overlooked. Dogwood’s oncology team in Marietta, GA shares what to watch for — and what to do the moment you notice it.

1. A New Lump or Bump

Not every lump is cancer — lipomas are common and usually benign. But any new lump that appears, grows, changes shape, feels hard or irregular, or causes discomfort deserves a fine needle aspirate. Never assume a lump is benign without testing it.

→ ACTION: Ask your vet to aspirate any new lump, no matter how small. If results are inconclusive, request a histopathology biopsy.

2. Unexplained Weight Loss

Cancer dramatically increases the body’s metabolic demands. Dogs may lose muscle mass and weight even while eating normally. Unexplained weight loss of more than 5–10% of body weight over a few weeks is always worth investigating.

→ ACTION: Weigh your dog monthly. If you can feel the spine or ribs more prominently than usual, schedule a vet visit.

3. Persistent Lameness or Difficulty Moving

Bone cancer (osteosarcoma) is common in large and giant breed dogs and presents as gradual lameness in one limb. Many owners attribute this to a sprain or arthritis — spending weeks treating the wrong condition.

→ ACTION: Lameness in a large-breed dog that doesn’t resolve within 2–3 days or is accompanied by swelling warrants X-rays — not just pain medication.

4. Changes in Bathroom Habits

Straining to urinate or defecate, blood in urine or stool, or unusual frequency can signal tumors of the bladder, prostate, colon, or rectum. These are easy to dismiss as infections or dietary changes — but when they persist, imaging is essential.

→ ACTION: If GI or urinary symptoms last more than a week or recur after treatment, ask for ultrasound or X-rays in addition to urinalysis.

5. Abnormal Odor

Unusual odors from the mouth, nose, or ears — beyond ordinary ‘dog breath’ — can signal tumors. Oral cancer often produces a distinctive smell due to tissue breakdown. Nasal tumors may cause nosebleeds and foul discharge.

→ ACTION: Schedule a dental exam if you notice persistent bad breath, especially with difficulty eating or facial swelling.

6. Swollen Lymph Nodes

Lymph nodes under the jaw, in front of the shoulders, behind the knees, and in the groin are normally small and difficult to feel. If they become palpably enlarged — firm, round lumps under the skin — lymphoma is a top concern. Lymphoma is the most common cancer in dogs and is highly responsive to chemotherapy when caught early.

→ ACTION: Learn where your dog’s lymph nodes are and check them monthly. Nodes that don’t shrink within 2 weeks require a vet visit.

7. Prolonged Lethargy or Personality Change

Every dog has an off day. But when a previously energetic dog becomes consistently withdrawn, loses interest in activities they used to love, or seems mentally flat for more than a week or two, something is wrong. Pain, nausea, and the systemic effects of cancer all cause changes that are easy to attribute to ‘just aging.’

→ ACTION: If your dog’s energy or personality has changed and doesn’t bounce back within a week, schedule a full physical exam and bloodwork.

8. Difficulty Eating, Swallowing, or Breathing

Oral tumors make eating painful. Chest masses cause coughing, breathing changes, or exercise intolerance. These signs are often attributed to dental disease or respiratory infections — but when they persist, imaging is essential.

→ ACTION: Coughing, gagging while eating, or labored breathing lasting more than a week needs X-rays — not just antibiotics.

Many cancers that are fatal in dogs are treatable — sometimes curable — when caught early. Annual bloodwork and physical exams, combined with an owner who knows their dog’s normal, are the best early detection tools available.

Noticed any of these signs? Contact Dogwood’s oncology team: (404) 609-1234 | info@dogwood.vet. Consultations, biopsies, cancer staging, chemotherapy, and palliative care — under one roof.