Possible Chemotherapy Side Effects & How To Help Your Dog Avoid Them

Whenever we treat cancer with chemotherapy our goal is that it prolongs our patient's life but doesn't compromise their quality of life.

Chemotherapy in dogs is much different than chemotherapy in people. We treat dogs much less aggressively and in doing so, we find that chemotherapy causes fewer side effects.

Can we promise that there won't be any side effects? No. However, if you're well-educated on the the most important (and earliest) signs that your dog isn't feeling well and have medications on hand to offer treatment as soon as possible, we can typically ensure that your dog will have a good experience with chemotherapy.

Please keep in mind that the information in this article applies to chemotherapy side effects. There are certain types of cancers that cause significant nausea, diarrhea, and inappetence if they are not in remission (not being treated effectively enough). In those cases it's the cancer (not the treatment) that's causing the patient to feel poorly and a protocol adjustment or change might needed to help them feel better. If your dog is feeling poorly and you're not sure if he's feeling bad from his disease or the treatment for his disease, talk to your care provider.

MDR1 Mutation

Some dog breeds have an increased risk of having a genetic mutation called MDR1 that makes them VERY sensitive to certain types of chemotherapy (and other medications). For these patients, the normal dose of some of our most common chemotherapy agents will make them very sick (best case) or be fatal (worst case).

If we need to treat a "problem breed" with a "problem drug" an oncologist will either substitute a different drug while we wait for test results (to find out if that patient has the mutation) or greatly reduce the dose of the possible "problem drug" to minimize the chance of making a patient sick (and add on anti-nausea meds, anti-diarrhea meds, antibiotics).

If the vet managing your dog's cancer is not an oncologist, they may not be aware that your dog is a "problem breed". Please read the MDR1 article to ensure your dog's breed is not on the list.

Gastrointestinal Upset (Nausea/Diarrhea)

GI side effects are most common 2-4 days after chemotherapy. We typically say that about 25% of dogs will develop nausea or diarrhea and 75% will not.

As a dog owner, what can you do to help your dog have the best experience with treatment? This is easy. You can treat the FIRST signs of nausea and diarrhea so that they resolve ASAP and your dog feels poorly for a few hours, OR you can try to prevent nausea by treating prophylactically.

TIP: If your dog is receiving chemotherapy, do not leave your vet's office without having anti-nausea and anti-diarrhea medication on hand. If your dog starts feeling sick at 10 PM on a Friday night, you need to be able to treat him at 10 pm to resolve his signs, not the next morning (when things will be worse). I recommend Cerenia for nausea and metronidazole for diarrhea.

Nausea - Option 1 (Monitor Closely):

(1) If you choose to monitor, you have to know that nausea is most likely to occur 2-4 days after receiving chemotherapy. When I'm talking about chemotherapy in this case, I'm talking about the type of chemotherapy that you go to your care provider for, not the type that you give at home.

(2) In this scenario, you must give Cerenia at the very FIRST sign of nausea. The goal is to PREVENT vomiting. Vomiting is the LAST sign of nausea. If you wait until vomiting, your dog will feel worse and become dehydrated. Oral medications are less likely to work if a dog is vomiting. At that point, an outpatient vet visit for subcutaneous fluids and injectable Cerenia is typically needed to help them recover quickly.

(3) Early signs of nausea include decreased appetite, drooling more than normal, lethargy, lip smacking, walking to the food bowl and walking away without eating. If ANY of these signs are noted, give Cerenia and continue for a few days.

TIP: If you are giving Cerenia, wait at least 30-60 minutes before offering food. If food is offered before Cerenia has had a chance to begin working, your dog will feel more nauseous (when we're nauseous the sight and smell of food makes things worse - dogs are the same!). Don't offer food to a dog that is overtly nauseous. Wait until they are looking/feeling better. The primary goal is always to resolve the nausea, not to get them to eat. They can go without food for a couple of days. If nausea is not resolving with oral meds, call your care provider for fluids and injectable medications (and to rule out other possible causes of not eating such as pancreatitis, kidney failure, etc).

Nausea - Option 2 (Treat Prophylactically):

(1) Some dog owners have a zero tolerance policy when it comes to nausea - they want to avoid it at all costs. If this is how you feel, do the following: Do not feed your dog the day of chemotherapy (especially with doxorubicin), ask your vet to give a Cerenia injection with chemotherapy, continue oral Cerenia at least 30-60 minutes before breakfast for the next four days. The only downside to this approach is that treatment will cost more because you're using more Cerenia and paying for injections.

(2) Make sure you're giving at least 2 mg/kg of Cerenia (the dose described on the box) - if your dog isn't receiving the correct dose, Cerenia will not work as well.

TIP: If your dog is not eating well despite Cerenia, other options include: adding ondansetron (another anti-nausea medication), adding Entyce (appetite stimulant), ruling out other underlying conditions that could cause decreased appetite (kidney failure, pancreatitis, progression of your dog's cancer, aspiration pneumonia, etc.).

Soft Stool and Diarrhea

If your dog is receiving chemotherapy and the stool becomes softer than normal, give the anti-diarrhea medication that you have on hand, which is typically metronidazole. Do not wait until your dog has liquid diarrhea (for vets, soft stool = diarrhea and warrants treatment). If soft stool is not treated it will often progress to liquid diarrhea and the patient will become lethargic, have a worse appetite and become dehydrated. Who wants that?

The recommendation is to treat at the FIRST sign of the stool changing. Remember that we want dogs to have a good experience with chemotherapy. Being proactive at home plays a significant role in achieving this goal.

Your care provider may recommend a five-day course of metronidazole or they may recommend giving it until the stool appears normal, then giving it for one additional day. If metronidazole is not resolving the soft stool after 2-3 doses, call your care provider for additional recommendations (tylan powder, psyllium, probiotics, etc.).

TIP: If your dog develops soft stool with a bit of frank blood the evening he returns from chemotherapy treatment, this is likely "stress colitis" and due to the stress of being away from home and in a different environment. It is not from chemotherapy but can be treated with metronidazole.

TIP: Don't change your dog's food or start a new supplement within the first few days of your dog's chemotherapy treatment. If either of these changes cause diarrhea, your care provider will have a very difficult time determining if diarrhea was due to chemotherapy or the diet change/new supplement. Discuss diet change with your care provider. If they support a diet change, make sure to time it appropriately and gradually change the diet over 2 weeks.

Changes in Cell Counts

After chemotherapy, your care provider will perform a CBC (complete blood count) to help determine how treatment has affected your dog's cell counts. We are primarily interested in the neutrophils (the white blood cell we monitor closely) but also platelets (they help us to clot blood). Injectable chemotherapy in dogs doesn't typically change red blood cell counts.

The neutrophils are typically their lowest 1 week after chemotherapy. For certain types of chemotherapeutics, these cells are at their lowest (called the nadir) two weeks after treatment.

The oncologist needs to determine the nadir to know if the dose of chemotherapy needs to be reduced.

If the CBC shows a very low neutrophil count the patient will be susceptible to infection, will need antibiotics, and will need a dose reduction the next time they receive that chemotherapy treatment.

During the days when the neutrophil count is very low, dogs can show signs of nausea and diarrhea. So, if your dog received chemotherapy 5-7 days ago and suddenly feels very poorly, reach for the phone to get your dog in for a CBC that day, then reach for the Cerenia and metronidazole.

At that CBC visit, your dog will also need to have his temperature taken to ensure he doesn't have a fever. If he has a fever AND a very reduced neutrophil count, hospitalization is recommended for IV fluids, IV antibiotics and other supportive care. Thankfully, this is considered rare (maybe 2% of cases receiving chemotherapy).

Other Lab work Abnormalities

Every chemotherapy agent is different. Some affect the kidneys, some the liver, one has the potential to affect the heart, one has the potential to irritate the bladder.

When receiving a particular chemotherapy agent, your care provider will explain the potential side effects associated with each agent and what (if any) changes you can monitor for at home. It's important that you tell your care provider if your dog isn't feeling well or isn't responding to the medications that you have for nausea/diarrhea - that's the only way they can help him feel better.


Palladia is an oral chemotherapy agent that is typically given at home on Monday, Wednesday and Friday. It was originally used to treat Mast Cell tumor but has been shown to be helpful for many types of cancers.

Unlike other types of chemotherapy, if your dog is receiving Palladia and isn't feeling well, the best thing to do is STOP Palladia, call your vet for advice, and give the appropriate treatment (metronidazole or an anti-nausea medication) until you hear back from them.

To increase the chance that Palladia is well-tolerated, most oncologists will make sure that their clients are not administering prednisone or an NSAID (carprofen, metacam, piroxicam, etc.) on the same day as Palladia. If given on the same day, it significantly increases the chance of significant gastrointestinal upset (nausea, diarrhea, poor appetite). In many cases patients cannot tolerate being on an NSAID and Palladia at all - every patient is different.

I also recommend that patients receive ondansetron (anti-nausea medication) 30-60 minutes before Palladia to decrease the chance of nausea.

Chemotherapy is an integral part of treatment for many types of cancers in dogs. Thankfully, it is often very well-tolerated. With the right knowledge of how to prepare for treatment, which medications to have on hand, which signs to look for, and what changes to make if a patient isn't feeling well, we can significantly increase the chances that our canine companions will have a good experience with treatment.

Dr. Lori Cesario

Board Certified Veterinary Oncologist

PS: I'm happy to now offer online oncology consultations. Learn more about how this service can help by visiting the Vet Cancer Consultants site.

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