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Chemotherapy is the administration of medications that are toxic to cancer cells. Dogs and cats generally tolerate chemotherapy much better than human patients do.

What is chemotherapy?

Chemotherapy drugs are compounds that are toxic to cancer cells. Chemotherapy is sometimes used by itself to treat certain types of cancer. In other cases, it is used in conjunction with surgery or radiation therapy where it may enhance the effectiveness of those treatments. Chemotherapy may be given intravenously (in the vein), by subcutaneous (under the skin) injection, or orally.


How does chemotherapy work?

Cancer cells generally multiply very rapidly. Most chemotherapy drugs work by damaging the ability of these rapidly growing cells to divide, eventually killing them.


What are the benefits of chemotherapy?

  • Chemotherapy is the most effective single treatment for some types of cancer, offering the best opportunity for remission and for preserving a good quality of life. A good example of this type of cancer is lymphoma (also called lymphosarcoma).
  • Chemotherapy is often recommended after surgical removal of a malignant cancer. The purpose of chemotherapy in this setting is not only to prevent recurrence of the cancer at the original site, but also to prevent spreading (metastasis). Examples of cancers in which chemotherapy is routinely used in this way are hemangiosarcoma of the spleen in dogs and malignant (cancerous) bone tumors in dogs
  • Chemotherapy may be administered to some animals while they are also receiving radiation therapy for the treatment of their cancer. Some chemotherapy drugs are effective in this situation because they increase the ability of the radiation to kill the cancer cells.
  • Occasionally, chemotherapy will be used alone for the treatment of cancers in which it is not possible to perform surgical removal or radiation therapy, or in cancers that have already metastasized (spread). In most of these cases, the goal of treatment is not to cure the cancer but rather to improve the patient’s quality of life temporarily by reducing pressure, bleeding, or pain.


More information

Generally speaking, the risk of a person becoming exposed to significant amounts of chemotherapy as a result of handling their pet is very low. We do not recommend changing your lifestyle or your pet’s as part of chemotherapy treatments (for example, do not ban your pet from sleeping with you if this is part of their normal routine). Some of the chemotherapy drugs (cyclophosphamide, carboplatin, and some others) that your pet may receive may be found in their urine or feces for 1 to 2 days after administration. Please see specific handouts for each drug to learn about the differences between medications. In general for cats, change the entire litter box once daily for the first 2 days after chemotherapy administration and be diligent about scooping waste throughout the entire time your cat receives chemotherapy (place waste in an outdoor receptacle). For dogs, try to bring them to a low-traffic area (for example, the back corner of the yard) for eliminations. If your pet has an accident in the house, wear gloves when cleaning it up. Wipe up the waste, then clean the area with a mild soap and water solution followed by a water rinse. Place these materials in a plastic bag and deliver it to an outside trash receptacle.

Repeated, long-term exposure to chemotherapy drugs can result in severe health problems. It is very unlikely that an individual occasionally administering chemotherapy pills to their pet would ever develop these problems, but precautions in handling medication should be used. ALWAYS wear gloves when handling chemotherapy pills, and wash hands thoroughly when administration is complete. Women who are breastfeeding, pregnant, or trying to conceive, as well as children and immunosuppressed individuals should never handle chemotherapy drugs. You can put the pills into a meatball to administer the pill (this usually works for dogs but not for cats), but try to make sure that your pet does not spit them out. NEVER split or crush the pills. For cats, pet pillers are an inexpensive, easy way to administer medications (ask your doctor where one can be purchased). Please ask your doctor or technician for a demonstration on how to pill your pet if you are not sure. If your pet spits out the pills, and they begin to “melt” or break apart, wear gloves and use paper towels when picking up the medication. Wipe the floor with a diluted soap and water solution and rinse with water if possible. Put the towels and medication into a plastic bag and call your doctor for advice about what to do next.

It is important for your veterinary oncologist or your regular veterinarian to examine your pet periodically after chemotherapy is over, usually at 1- to 2-month intervals. This will allow potential problems, such as recurrence of the cancer, to be detected before they become too advanced. Treatment options will be more numerous and have a greater potential for success when problems are identified early.

Finally, it is important for the owners of dogs and cats receiving chemotherapy to realize that the cancers we treat are rarely cured. Almost all of our patients ultimately have recurrence of their cancers. However, it is vital to understand that most cats and dogs receiving chemotherapy have an excellent quality of life both during and after treatment. It is often possible to provide many additional months, or sometimes even years, of happy life with chemotherapy. The vast majority of owners tell us that they have no regrets about their decision to pursue chemotherapy for their pet.

The length of a particular course of chemotherapy will vary depending on the disease being treated. The most common cancer treated with chemotherapy is lymphoma. Although chemotherapy for this disease is often successful, owners of cats and dogs with this type of cancer should realistically expect that their pets will need some form of chemotherapy intermittently for the rest of their lives. The course of treatment for other types of cancer is usually much shorter, generally 3 to 5 months. The specific length of your pet’s individual course of treatment will be discussed in detail with you.

Treatment of cancer with chemotherapy can be costly. It involves the same drugs used to treat human cancer patients, and many of these are expensive. In addition, your pet will benefit from the expertise of several highly trained health care professionals. The exact cost of chemotherapy varies with the size of the animal, the number of treatments, and the drugs being administered. The projected cost of your pet’s individual treatment will be discussed in detail with you. Keep in mind, the cost of an individual drug may change during treatment because of the constant changes we see in drug costs from suppliers.

One or more cancer specialists will examine your pet directly and consult in detail with you and your regular veterinarian. These specialists will determine whether chemotherapy will be useful for your pet’s cancer. If chemotherapy is given to your pet, a veterinary oncologist (specialist in cancer medicine) will carefully plan the course of therapy. Treatment for each patient is individually tailored, although specific chemotherapy protocols consisting of several different drugs are followed for different types of cancer.

The large majority of chemotherapy drugs are given by intravenous injection. A blood sample is drawn first, and the white blood cell count, red blood cell count, and platelet count are checked to make sure that it is safe to proceed with treatment. In some cases, your pet may not receive treatment due to a low white or red blood cell count. If it’s safe to proceed, the drug itself is then administered through an intravenous catheter. Owners should expect that this process will take 2 to 3 hours.

There are risks involved with any type of treatment for cancer. Some normal cells will be injured and killed by the chemotherapy drugs. Side effects may be apparent because of these normal cells being killed. However, these side effects are usually outweighed by the benefits of eliminating the cancer cells.
Dogs and cats generally tolerate chemotherapy much better than human patients do. The two side effects encountered most commonly in canine and feline patients receiving chemotherapy are toxicity to the gastrointestinal tract and toxicity to the bone marrow. Normal cells in both of these areas divide very rapidly, and as such, are more susceptible to the toxic effects of the chemotherapy. When the cells lining the gastrointestinal tract are affected, the result may be vomiting or diarrhea. Most patients will experience this side effect at least once or twice during their course of chemotherapy treatment, but the symptoms are usually mild and can be overcome with supportive care at home.

White blood cells are cells of the immune system responsible for fighting infection. The bone marrow produces these cells (called progenitor cells). If these progenitor cells are damaged, the patient’s white blood cell count may drop low enough to result in an increased susceptibility to infection. Even bacteria to which a patient would normally be resistant can cause serious illness in this situation. White blood cell counts of all canine and feline chemotherapy patients are monitored carefully; however, occasionally a cat or dog receiving chemotherapy will develop a life-threatening systemic infection. The only way to successfully treat these infections is to admit the patient to the hospital and administer intravenous fluids and antibiotics.

Hair loss in cats and dogs receiving chemotherapy is usually very minor, with some notable exceptions. If you have a Poodle, Old English sheepdog, Schnauzer, Puli, Lhasa Apso, Shih Tzu, Bichon Frise, Terrier, or Maltese, you should expect your pet to lose a significant amount of hair during the initial stages of chemotherapy. However, the hair that is lost will grow back after your dog’s course of chemotherapy has been completed or once treatments are being administered less frequently. Hair may need to be clipped frequently during chemotherapy to identify veins. This hair may be slow to regrow. Cats usually do not lose hair, although many will lose their whiskers.

Some chemotherapy drugs can be extremely irritating to the subcutaneous tissues if they are able to leak outside of the vein during injection. Examples include the chemotherapy drugs vincristine, vinblastine, adriamycin, and mustargen. Severe swelling, ulceration, and inflammation can be seen. However, this complication is rare because all chemotherapy drugs are carefully administered through catheters placed in the vein.

Veterinary Professionals