Potential side effects of chemotherapy use in pets

The goal of veterinary chemotherapy is to give pets more good quality life than they would have without chemotherapy.

Research Authors


The goal of veterinary chemotherapy is to give pets more good quality life than they would have without chemotherapy. We use similar drugs to those used in chemotherapy for people, however we use lower doses specifically to reduce the risk of side effects. However chemotherapy drugs target rapidly dividing cells throughout the body, so there is a risk of pets developing side effects. Usually the side effects are mild and short lived, and can be managed at home without needing a recheck in clinic. More severe side effects (requiring hospitalisation) are rare, occurring in less than 10% of patients.

Bone marrow suppression

Many chemotherapeutic agents affect the rapidly dividing cells within the bone marrow. The cell type most commonly supressed is the neutrophil. Neutrophils are involved in the body’s “first line” defence against infection. A decrease in the number of circulating neutrophils is called neutropenia. Neutropenia usually occurs 7-10 days after most chemotherapy administrations. Neutrophil levels will be checked before each chemotherapy dose, and the chemotherapy may be delayed if neutrophil levels are too low (to avoid suppressing the neutrophil levels further). Neutropenic pets may be more susceptible to infections, which can in rare cases lead to sepsis. Signs of sepsis include: sudden lethargy, loss of appetite, increased temperature (fever). Sepsis is rare however can be life threatening and requires hospitalisation for IV antibiotics and monitoring. Other bone marrow cells can also become suppressed (eg platelets), particularly with repeated chemotherapy doses. We monitor blood cell levels throughout treatment and will alter the treatment if any cells are becoming too low.

Gastrointestinal Discomfort

Patients can experience some form of gastrointestinal discomfort after chemotherapy. This usually occurs 2-5 days after the chemotherapy is given. Usually these side effects are mild and self-limiting and do not significantly impact the pet’s quality of life. Often management involves changing to a bland diet, offering smaller meals throughout the day, and/or the use of supportive medications. We usually dispense medications for you to have on hand in case you encounter these side effects. If these side effects are severe then hospitalisation for IV fluids may be required (this is rare). The most common gastrointestinal side effects are:

  • Nausea, appetite loss and vomiting: Signs of nausea include: lip-smacking, drooling, eating grass (unless this is normal for your pet), or loss of appetite or decreased interest in food. One or two single vomits may occur. We will usually dispense an anti-nausea medication (ondansetron) for you to use straight away if vomiting occurs.
  • Diarrhoea: This is commonly transient and usually lasts for less than 24-48 hours. This can be accompanied by signs of colitis (straining, small amount of blood/mucous in the stools). If diarrhoea occurs we recommend immediately starting an anti-diarrhoeal paste (such as Promax), and this is usually all that is required to resolve the diarrhoea.

Tissue Damage

Some chemotherapy drugs can cause severe tissue reactions if they extravasate (leak outside the vein) during administration. This is why we take great care when placing the IV cannulas used for chemotherapy, and sedate some animals for some drugs (eg doxorubicin).

Allergic Reactions

Some chemotherapy drugs can cause allergic reactions. If we use a drug that can cause a reaction we give other medications (eg anti-histamines) at the same time to reduce the risk.

Hair Loss (Alopecia)

Pets rarely lose their hair while undergoing chemotherapy. The pets most at risk are animals that have hair that continually grows (ie Poodles, Old English Sheepdogs, West Highland White Terriers). Cats may lose their whiskers. If a pet loses their hair during treatment, it almost always grows back once treatment has been completed.

Drug-specific side effects

Some of the chemotherapy drugs are associated with specific side effects. If we use these drugs, we will monitor carefully for these side effects with specific tests.

Heart Damage: Doxorubicin can cause a cumulative cardiotoxicity in dogs. Most dogs will not have any heart side effects if their total lifetime dose is less than 180-240mg/m2 . If there is underlying heart disease the risk is increased, so we may check the heart with an echocardiogram before giving the first dose of doxorubicin, particularly in breeds with a predisposition for heart diseases (such as Boxers and Dobermans).

Kidney Damage: Some drugs such as doxorubicin in cats, cisplatin in dogs, or toceranib in both, can cause renal damage. This is monitored with periodic blood and/or urine tests. If renal damage occurs, usually we can identify it and change our treatment before it causes renal failure.

Liver damage: Lomustine can cause liver damage, particularly with repeat doses. We monitor for this with blood tests before each dose and will often use a liver supplement to protect the liver for 1-3 weeks after each dose is given.

Cystitis: Cyclophosphamide can cause sterile haemorrhagic cystitis. Dogs with this condition have bladder pain which causes them to strain to urinate frequently. This condition can be difficult to manage so we take several steps to prevent it from occurring. These steps include: only giving cyclophosphamide in the morning (so it does not sit in your pets bladder overnight), giving a diuretic at the same time as the cyclophosphamide to increase urination and reduce the amount of time the metabolite is in the bladder, and periodically checking the urine for microscopic traces of blood. If there are any signs of impending cystitis we will change drugs in order to reduce the risk of cystitis developing.