Dog chemo side effects: what is normal, what to watch, and when to call
Your dog starts chemotherapy next week, and you are afraid of what it will do. That fear is normal. The good news is that chemotherapy for dogs is not the same as chemotherapy for people. Here is what vets say is normal, which days to watch, and the signs that mean you call now.
When to call the vet now
Read this part first. Then write your oncology team’s phone numbers where you can see them.
The Ohio State University oncology team tells owners to call immediately if the dog:
- skips two or more meals;
- has any vomiting or diarrhoea;
- is lethargic, or has a rectal temperature over 102.5 °F (39.2 °C);
- has any other sign that worries you.
The University of Missouri Veterinary Health Center adds these signs:
- your dog does not respond to you;
- vomiting or diarrhoea more than 3 times an hour, or 3 times a day;
- blood in the vomit, the poo or the urine;
- your dog “just doesn’t seem to be feeling well”.
Ohio State also says to call if your dog licks the injection site, or if the site is red or swollen. Your own team may give you different rules. Their rules come first. The Dog Cancer Care Journal has these signs on page 2, with space for your team’s numbers and rules.
Most dogs cope well
The 2026 AAHA Oncology Guidelines say the main goal of chemotherapy for pets is to keep the best possible quality of life. They say most protocols “are well tolerated, with side effects that are typically mild and manageable.”
The numbers agree. The University of Missouri says more than 80 percent of patients cope well enough to be managed at home. Fewer than 10 percent need a hospital stay for side effects. Ohio State says that in most cases the side effects are mild, are easy to treat, and do not affect quality of life.
Why the difference from people? The aim is different. The University of Missouri says the goal is longer life, balanced with a good quality of life. Ohio State says that in general, chemotherapy drugs control cancer rather than remove it for good.
The common side effects
A 2026 study from the University of Surrey and AURA Veterinary asked owners to record signs every day for 21 days after a treatment. The dogs had lymphoma, mast cell tumours or sarcomas. Over 504 daily records, the most common signs were:
| Sign | Share of daily records |
|---|---|
| Tiredness (fatigue) | 16.1% |
| Drinking more than normal | 13.5% |
| Eating less, or not eating | 12.9% |
| Diarrhoea | 5.2% |
| Vomiting | 4.0% |
Most of these signs were mild or moderate. Each dog had a median of 3 different signs over the 21 days. Pain and laboured breathing were less common.
The AAHA guidelines list the same groups: nausea, vomiting, a poor appetite, diarrhoea and low blood counts. They add that these can lead to muscle and weight loss. So weigh your dog each week on the same scale.
Hair loss is rare. Ohio State says it happens mainly in breeds with coats that keep growing, such as poodles and shih tzus. Chemotherapy slows the regrowth of a clipped coat.
Some drugs have their own side effects. Ohio State gives two examples: one drug can affect the heart, and another can irritate the bladder. Ask your oncologist which special signs to watch for with your dog’s drugs.
Which days to watch
Side effects follow a pattern after each treatment. It helps to know it.
- Days 2 to 5: the stomach and gut. Ohio State says gut side effects most often come in this window. The University of Missouri says they usually start within 3 days and last 3 to 5 days.
- Days 7 to 10: blood counts. The University of Missouri says low blood counts come about 7 to 10 days after a treatment. Most dogs show nothing. Ohio State says a low white cell count can come with fever, tiredness or less appetite.
- Before the next treatment: a blood test. Ohio State says a blood count is done before each treatment. If the counts are low, the treatment can move by a few days.
Your dog’s pattern may be different. After two or three treatments, a daily record shows your dog’s own pattern. Then you know which days to stay home, and which days are good for a long walk.
What to record each day
Vets see your dog for a short time. You see your dog every day. Your record is part of the care.
The Surrey team asked owners to record seven things each morning, at about the same time: activity or tiredness, appetite, drinking, poo (diarrhoea, constipation, blood or mucus), vomiting, breathing and pain. Owners found it “very easy”. One owner took the notes to the next visit, and the vet said the panting and tiredness were “totally normal”.
The study also found a problem. Some owners did not know what to compare with: yesterday, or before chemotherapy. Compare with normal for your dog before treatment. Write that normal down before the first treatment: meals, water, walks, sleep, poo and weight.
A simple daily line:
- The date, and the number of days since the last treatment.
- A score for each sign: 0 normal, 1 mild, 2 moderate, 3 severe.
- Anything else: blood in the poo, a cough, a temperature.
- Did you tell the vet?
The daily symptom diary in the Dog Cancer Care Journal has these columns, with a weekly quality of life check and a blood test log. Before a visit, you can also make a one-page summary with the free vet visit summary tool.
Safe at home after a treatment
Ohio State says most chemotherapy drugs leave the body in urine and poo for several days to a week. Some take several weeks. Simple steps keep your family safe:
- Wear gloves to clean up urine, poo or vomit.
- Use paper towels or pads, then throw them away.
- Wash soiled bedding on its own, in hot water.
- Wash your hands after, even with gloves.
If you give chemotherapy pills at home, Ohio State says to wear gloves, give them exactly as prescribed, and never crush or split them or open capsules. Ask your team how many days to take care, and who in your home should not clean up.
How vets help with side effects
You do not have to wait it out. Ohio State says pets usually go home with medicines to prevent and control stomach side effects, with clear instructions. The AAHA guidelines say to address a poor appetite early, so the dog can stay on its treatment plan. Give only what your vet prescribes, exactly as the label says. Do not give human medicines.
If side effects are hard for your dog, tell your oncologist. The plan can change. Ohio State says that when the drugs no longer control the cancer, the vet talks with you about comfort. For that time, see our guide on how to know when to put your dog down.
Keep it all in one place
Chemotherapy brings a lot of dates, drugs, numbers and questions. Keep one record: the plan as the vet writes it, each treatment date, the daily signs and the blood results. If your dog’s appetite is the problem, read dog not eating after chemo. Before the first visit, read questions to ask your dog’s oncologist.
The Dog Cancer Care Journal is a fillable PDF with all of these pages. It never chooses a treatment and never names a dose. Your vet and oncologist decide; you keep the record.
If you need to plan for the end, read what to do when your dog dies at home and how to know when to put your dog down.
Questions
Do dogs suffer during chemotherapy?
Most do not. Ohio State University says pets do not get the same side effects as people, and severe side effects that need a hospital stay are uncommon. The University of Missouri says fewer than 10 percent of patients need a hospital stay for side effects. Hair loss is rare, mainly in breeds with coats that keep growing, such as poodles.
How will my dog feel after chemo?
Many dogs feel normal. Some are tired, eat less or have a loose stomach for a few days. Ohio State says stomach and gut side effects most often come 2 to 5 days after a treatment. In a 2026 University of Surrey study, tiredness was the most common sign owners reported, and most signs were mild or moderate.
Is it safe to be around my dog after chemo?
Yes, with simple care. Ohio State says most chemotherapy drugs leave the body in urine and poo for several days to a week, and some for longer. Wear gloves to clean up urine, poo or vomit. Wash bedding on its own in hot water. Ask your team how many days to take care for your dog's drugs.
When should I call the vet during chemo?
Ohio State tells owners to call at once if the dog skips two or more meals, has any vomiting or diarrhoea, is lethargic, or has a rectal temperature over 102.5 °F. The University of Missouri adds blood in vomit, poo or urine, and a dog that does not respond to you. Your own team's rules come first.
Why does my dog drink more water on chemo?
Drinking more was the second most common sign in the 2026 Surrey study. Most of those dogs also took a steroid, and the authors say the steroid may explain much of the thirst. Write down how often you refill the water bowl, and tell your vet. Ask your vet before you change your dog's water.
Can chemo side effects show up a week later?
Yes. The University of Missouri says low blood counts come about 7 to 10 days after a treatment. Most dogs show nothing. Ohio State says a low white cell count can sometimes come with fever, tiredness or less appetite. That is why a blood test often comes before each treatment.
Sources
- AAHA: 2026 AAHA Oncology Guidelines for Dogs and Cats
- The Ohio State University Veterinary Medical Center, Oncology Service: Chemotherapy for Cancer Treatment in Animals (PDF)
- University of Missouri Veterinary Health Center: Chemotherapy
- Harris J and others (2026). Development and Testing of an Owner-Reported Outcome Measure of Clinical Signs and Quality of Life in Dogs Treated With Chemotherapy. Veterinary and Comparative Oncology 24: 80-94
This article helps you organise care. It does not replace advice from your vet.
