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Chemotherapy For Dogs & Cats

Worried About Chemotherapy?

Most of us have direct experience of someone close to us that has been given chemotherapy for treatment of  human cancers. Chemotherapy is a very emotive word and may generate expectations of unpleasant side-effects  such as nausea or vomiting.  

In reality, we use chemotherapy in a rather different way from our counterparts in human medicine. Human  beings treated with chemotherapy can rationalise that short-term side-effects, however severe, are ‘put up with’  in the expectation that the result will possibly be a cure of disease.

However in our veterinary patients, to whom  we obviously cannot rationalise such an approach, we take the view that it is unethical to provide treatment  associated with such side effects as routine and we use the approach of trying to prolong quality of life rather  than length of life, no matter what the consequences.  

The practicalities of this approach are that we use less ‘aggressive’ chemotherapy regimes, and consequently we  see side effects uncommonly, with notable side effects occurring only in about one in every five or so patients. The trade off is that we do not see anywhere near the ‘cure’ rate seen in people, and our realistic expectation is to achieve remission (which is resolution of all  clinical signs of the tumour as it shrinks to an undetectable level) and to restore your pet to a normal quality of life, for as long as is practical. 

What is chemotherapy?

Chemotherapy drugs attack rapidly dividing cells such as those found within tumours which grow in an  uncontrolled way. Most means of treating cancers with chemotherapy involve giving regular doses of a  combination of drugs – with one or two exceptions, combinations have an ‘additive’ effect and help us treat  tumours more effectively – either in oral tablet form or as intravenous injections or infusions. Chemotherapy may  be combined with surgery or with radiotherapy, or sometimes both.

Each time a chemotherapy drug is given, a proportion of tumour cells are killed, but remaining cells start to  ‘repopulate’ in between doses of chemotherapy.  

There are other normal cells within the body that also rapidly divide, especially the gut and the bone marrow.  The bone marrow produces red blood cells (which carry oxygen), white blood cells (which fight infections and  keep under control bacteria which normally populate some areas of the body) and platelets (which help the  blood to clot). Of these the white blood cells and sometimes platelets are the most sensitive to the effects of  chemotherapy.

Fortunately, these tissues recover more rapidly than tumour cells. We give chemotherapy in a  way that ‘kicks the tumour cells whilst they are down’ but gives sufficient time for the bone marrow and gut cells  to recover. On average this is about once a week, but this may vary with some drugs. We monitor the bone  marrow by taking a blood sample each time your dog or cat is due to receive chemotherapy and checking a  haematology (sometimes known as a CBC). We do this to make sure that the bone marrow has recovered from  the previous dose. 

Will your cat or dog experience any side effects?

Side effects are uncommon in dogs and cats having chemotherapy and most of them can be avoided by pre-empting problems before they arise. The single most important factor in doing this is regular blood test  monitoring prior to each chemotherapy dose being given

White blood cell numbers (myelotoxicity) 

The most worrisome side-effect of chemotherapy for dogs and cats is one that it is unlikely that you, and more importantly your  pet, will notice, and that is of a very low white blood cell count developing. We can usually avoid this situation  developing by closely monitoring the white blood cell count. 

If white blood cell levels have not returned to  acceptable levels by the time the next chemotherapy sample is due, then we simply delay this until it has (usually  3-7 days). Do not be alarmed if your dog or cat’s chemotherapy needs to be delayed in this way on occasion – it is actually a sign that we are treating the tumour reasonably aggressively, and it is better to do this than to give an insufficient dose.

If the white blood cell count is very low then we may advise that your pet receives a short course  of antibiotics. This is because when pets have very low white blood cell counts they become prone to developing  infections, usually caused by bacteria that they normally carry in the gastrointestinal or genital tract or their skin  (rather than being from the environment or other pets).

The lowest white blood cell count usually occurs about  5-10 days after administration of chemotherapy. If your dog or cat seems unwell at this time after chemotherapy,  please let us know. 

Gastrointestinal side effects

Gastrointestinal side effects are rare. Usually the most we will see is a short-lasting decrease in appetite for 24-48  hours after administration of chemotherapy.

Vomiting and diarrhoea are much rarer signs and are not expected  in the majority of patients. If you do see these occurring in your cat or dog, please let us know and we will prescribe  some medication to reduce or dramatically shorten this; we do not routinely prescribe anti-sickness drugs at  home for chemotherapy patients because this is seen so uncommonly.

Side effects of corticosteroids

Many dogs and cats with cancer may need to receive corticosteroids such as prednisolone or dexamethasone.  It is common, especially in dogs, to see an increase in drinking, urinating and appetite with these. This is not a  sign of kidney disease. Do not restrict water and allow plenty of opportunities to urinate. Giving the last dose in  the early evening may help with this.

Be careful that dogs do not scavenge human table scraps and from bins.  Other side effects that may be seen include some muscle wastage (usually around the head), slow hair growth,  slow wound healing and sometimes muscle weakness – these side effects reverse as the drug therapy is withdrawn.  

Other rare side effects

Some side-effects are specific to certain chemotherapy drugs and if your dog or cat is receiving the following you  should be aware of:  

Cyclophosphamide (Endoxana)

Occasionally, dogs and cats will develop a form of sterile cystitis (inflammation of  the bladder). If you notice signs of discomfort urinating or any blood in urine, please do not give any more of  this medication and let us know. 

We will probably advise that a urine sample is checked for other forms of cystitis  and may advise some anti-inflammatory / spasmodic medication. Signs will usually abate after discontinuation  of the drug.


In dogs, with cumulative doses of this drug there is a slight risk of heart muscle disease developing  and we may perform some testing to exclude the possibility of undiagnosed heart disease before considering  this drug. It is very rare to see cardiac side effects at the doses we use. In cats, very rarely kidney toxicity can be  seen, and is again something that we will be monitoring for. 

How to administer chemotherapy drugs safely 

Always keep chemotherapy drugs that are to be given at home in a secure place away from children and in their original container. You should administer any tablets whole (it is OK to give them in a bit of food but do  not crush or split them or simply put them in the food – you must be sure they have been taken) and wear  disposable gloves.

You should minimise your exposure to these drugs and to the metabolites of them (simple exposure is unlikely  to do you harm, but we want to eliminate regular exposure to these powerful agents).

Wear gloves if having to  clear up vomit, faeces or urine for 6-7 days after administration of any drugs and wash hands thoroughly after  any contact with body fluids including saliva during this time. There is minimal risk to your other pets. All family  members should be instructed to do the same. Faeces should be ‘double-bagged’ and disposed of normally.  Accidents on wipeable surfaces should be cleaned with household bleach if possible.

Oncology out-patient clinic

It is important that a complete blood count (haematology) is assessed prior to each chemotherapy being given.  If you are travelling a long distance it may be preferable for this blood sample to be performed at your own vets  provided that the results are available for us to review prior to your travelling for chemotherapy.

Please ensure  that the blood sample is not taken more than 48 hours before chemotherapy is due and that your vets send us  a copy of this in good time.

It is sensible to phone and check that we are happy with the results of this before  travelling. If there will be a delay of more than 48 hours in receiving results or if you are not travelling far, then  it is best that we perform the haematology test on the day of your appointment and this takes about half an  hour.

If all is OK, they can then be admitted for chemotherapy administration and will stay for about an hour to an hour-and-a-half.