Signs of cancer in dogs

The idea that our four-legged best friend might be sick is terrifying enough without the dreaded diagnosis of cancer. But just as with humans, spotting the signs early and getting your pup to the vet for a treatment plan is key. Whether you’ve found a fatty tumour on your dog or you’re suspicious that you’ve got a case of dog skin cancer on your hands, keep an eye out for these signs of cancer in dogs to help you identify a problem early.

By Alyssa Jung. All images: Shutterstock.


If your dog collapses, get to the vet immediately. Collapsing, weakness and general lethargy (not greeting people at the door like usual or less interaction) are common signs of cancer, says Jake Zaidel, DVM, of Malta Animal Hospital in upstate New York. “I see this particularly in large breed dogs – even if they fall down and seem better the next day, bring them in because it could signal a tumour of the spleen,” says Dr. Zaidel.


Coughing doesn’t automatically signal cancer; for example, small breed dogs tend to develop coughs because they have windpipe problems. “If the dog coughs once or twice, it’s of no concern, but if it continues to cough for more than a few days, that’s a concern and could signal lung cancer,” says Zaidel.

Weight loss

Weight loss is the number-one dog cancer symptom Dr. Zaidel says he sees. It’s often the sign of a gastrointestinal tumour. “I’ve had a lot of dogs stop eating because of gastrointestinal tumours, so they lose weight very rapidly,” he says. Cancer can also cause dogs to lose weight while maintaining their regular appetite. If you notice your dog shedding weight, either rapidly or slowly, make an appointment with your vet.

Mouth changes

Sores, lumps, a strange odour, bleeding or a change in gum colour can be a sign of oral cancer, particularly in older dogs. This cancer sign in dogs often goes unnoticed for too long. “We commonly find visible oral tumours because people don’t examine their pet’s mouth,” says Dr. Zaidel. “Many oral tumours can be really devastating because people don’t find them until it’s really advanced.” He also suggests brushing on a regular basis.


Nosebleeds are never normal, says Dr. Rocha. “With an older dog, a nosebleed is particularly worrisome. It can be a sign of cancer in the nose,” he says. “With younger dogs, I would worry more about something like a foreign object stuck up there before cancer.”

Diarrhea or changes in bathroom habits

Occasional diarrhea usually isn’t a sign of cancer in dogs, says Dr. Rocha, but if it persists or gets worse, get your dog to the vet. Constantly begging to go out to go to the bathroom, difficulty peeing/moving bowels, vomiting or blood in the urine or stool are also potential dog cancer symptoms, according to


Persistent discharge from the nose or eyes is cause for concern, says Dr. Zaidel. Nasal discharge is a common sign of facial tumours and eye discharge can signal an eye tumour.


Seizures can be a sign of brain tumours and are typically seen in older dog cancer patients, says Dr. Zaidel. If you start to notice sudden and uncontrolled bursts of activity, like champing and chewing, jerking of the legs or foaming at the mouth, your dog could be experiencing seizures and you should see a vet immediately, according to

Skin changes

“Every lump, bump or skin change should be checked,” says Dr. Zaidel. “It could be benign or cancerous, but it’s always easier to treat the earlier it’s caught.” Feel for bumps, lumps or swelling as you pet your pooch. If you notice something iffy, don’t delay – there’s no way to distinguish between a lump that’s benign or malignant without taking a sample. Also, pay attention to any sores that won’t heal or lesions that seem itchy or painful.

Weight gain

Sudden weight gain or bloating can be a sign of cancer in dogs. If your dog is eating less but seems to be bulking up, take a trip to the vet, says Rocha. A sudden spike in appetite also warrants a visit.

General pain or discomfort

“Pain is a rather substantial sign of cancer,” says Zaidel. If your dog whines or cries out when you pat her tummy or pick him up, call your vet. Mouth tumours may cause noticeable discomfort when eating.