Cancer is the #1 cause of death in older dogs — and most owners miss the early signs.
They seem small at first: a little tiredness, a lump that doesn’t go away, maybe a strange smell. But behind these symptoms could be something far more serious.
In this guide, you’ll discover 10 hidden signs your dog might have cancer — based on real veterinary advice.
Some may surprise you. #3 is the one most people ignore.
Let’s get started and protect your best friend while there’s still time.
10. Lethargy That Won’t Go Away
Is your dog sleeping more than usual? Moving slower? Not excited for walks?
These may seem like signs of aging — but persistent, unexplained fatigue is often an early red flag for cancer in dogs.
🩺 Why It Matters:
While dogs naturally slow down with age, veterinarians warn that sudden or ongoing low energy can be a symptom of lymphoma, bone cancer, or internal tumors affecting vital organs like the spleen or liver.
According to the American Kennel Club (AKC), lethargy is one of the top early signs seen in dogs later diagnosed with cancer. Similarly, PetMD notes that “low energy, decreased interest in activities, and a general sense of apathy can indicate an underlying illness, including cancer.”
⚠️ Watch for:
- Longer naps or constant sleeping
- No interest in toys or walks
- Delayed responses or slow movement
- Needing help getting up or navigating stairs
✅ What To Do:
Track your dog’s energy levels for a few days. If the fatigue persists, contact your vet and request a full blood panel and physical exam. Catching cancer early gives your dog the best chance at treatment.
📌 Note: Don’t just assume it’s old age — especially in dogs under 8. Always rule out medical causes first.
9. Unusual Lumps or Swelling That Doesn’t Go Away
Have you noticed a new lump or bump on your dog? Don’t assume it’s harmless.
Not all lumps are cancer — but those that persist, grow, or change shape can be signs of something more serious.
🧪 Why It Matters:
Many cancers, like mast cell tumors, soft tissue sarcomas, or liposarcomas, begin as small, painless lumps under the skin. While some are benign (like fatty tumors), malignant ones can spread quickly if left untreated.
According to the American Veterinary Medical Association (AVMA), any new or changing lump should be examined by a veterinarian, especially if it’s firm, irregular, or growing over time.
PetMD also emphasizes that “lumps that don’t go away within a week or two — or grow in size — should be checked immediately.”
⚠️ What to Look For:
- Swellings on the neck, legs, chest, or belly
- Lumps that feel hard or rubbery
- Lumps that bleed, ulcerate, or ooze fluid
- Growths that seem to appear suddenly
✅ What To Do:
Don’t guess. Ask your vet for a fine needle aspirate (FNA) or biopsy to identify whether the lump is benign or malignant. Early removal can save your dog’s life and reduce the chance of spread.
📌 Tip: Check your dog weekly with a gentle “hands-on” exam during grooming or playtime. Catching lumps early makes all the difference.
8. Unexplained Weight Loss
If your dog is losing weight without a change in diet or activity, it’s not normal — and it could be cancer.
⚠️ Why It’s Concerning:
Sudden or gradual weight loss — especially when your dog is eating normally — can be a sign of cancer affecting the gastrointestinal tract, liver, or pancreas. These cancers interfere with nutrient absorption, appetite, or metabolism.
According to PetMD, unexplained weight loss is one of the top signs vets look for during cancer screenings.
The American Kennel Club (AKC) also highlights that even a 10% weight loss in a short period is a red flag and should prompt immediate veterinary evaluation.
🧭 What To Watch:
- Noticeable rib visibility or spine showing
- Looser skin or muscle wasting
- Collar or harness suddenly loosening
- No change in feeding or exercise routine
Even in overweight dogs, unexpected weight loss can mask deeper health issues.
✅ What To Do:
Ask your vet for a physical exam, blood work, and possibly an abdominal ultrasound. Many GI cancers (like lymphoma or adenocarcinoma) show no obvious symptoms at first except weight loss.
📌 Pro Tip: Weigh your dog monthly at home or during grooming visits. Catching subtle changes early can lead to quicker diagnosis and better outcomes.
7. Bleeding from the Nose, Mouth, or Rectum
Bleeding without injury is never normal — and in many cases, it could be a sign of internal cancer.
🚨 Why It’s Dangerous:
Spontaneous bleeding may indicate hemangiosarcoma (a fast-spreading cancer of the blood vessels), or tumors in the nose, gums, or gastrointestinal tract. These often go unnoticed until they bleed — and by then, they may already be advanced.
The American Veterinary Medical Association (AVMA) warns that bleeding from body openings is among the more serious red flags that require immediate attention.
Veterinary Cancer Society also reports that hemangiosarcoma is often diagnosed only after sudden internal bleeding episodes, making early detection critical.
🔍 Signs to Watch:
- Blood in your dog’s saliva or when chewing
- Nosebleeds without trauma (especially in older dogs)
- Blood in stool or black/tarry feces
- Small pools of blood while sleeping or resting
Bleeding may come and go — but even one incident without explanation deserves a vet visit.
✅ What To Do:
Get a full blood panel, X-ray, and abdominal ultrasound as soon as possible. If internal bleeding is suspected, emergency treatment is often necessary.
📌 Don’t wait to see if it happens again. Cancer-related bleeding tends to recur — and it can escalate fast.
6. Persistent Coughing or Difficulty Breathing
A lingering cough isn’t just a cold — it could signal something much more serious.
If your dog is wheezing, coughing at night, or struggling to breathe, it may be time to investigate deeper.
🩺 Why It’s Serious:
Chronic coughing can be a sign of lung cancer, heart-related tumors, or cancer that has metastasized to the lungs from another part of the body. It’s especially dangerous if it appears suddenly in a previously healthy dog.
According to PetMD, a cough that lasts more than a few days, especially if it worsens over time, is one of the most common early signs of pulmonary cancer in dogs.
The Veterinary Cancer Society also notes that labored breathing and coughing may be the only signs until the disease is advanced.
🔍 What to Look For:
- Dry, hacking, or honking cough (not kennel cough)
- Wheezing or noisy breathing
- Fatigue after mild activity
- Bluish gums or tongue (lack of oxygen)
- Shortness of breath while lying down
These symptoms can overlap with heart disease, making a diagnostic workup essential.
✅ What To Do:
Your vet may recommend:
- Chest X-rays
- Blood oxygen level testing
- Bronchoscopy or CT scan if cancer is suspected
📌 Early treatment matters: Lung tumors can be surgically removed if caught early, improving survival and quality of life.