Merck Manual

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Pet Owner Version

Drugs Used to Treat Cancers and Tumors


Dawn Merton Boothe

, DVM, PhD, Department of Anatomy, Physiology, and Pharmacology, College of Veterinary Medicine, Auburn University;

Lisa G. Barber

, DVM, DACVIM-Oncology, Cummings School of Veterinary Medicine, Tufts University;

Kristine E. Burgess

, DVM, DACVIM, Cummings School of Veterinary Medicine, Tufts University

Reviewed/Revised Jul 2011 | Modified Nov 2022

Antineoplastic (antitumor) chemotherapy is often used to treat dogs and cats, and it is routinely used for selected tumors in horses. Antineoplastic drugs can be grouped into general categories, based on their biochemical method of action: alkylating agents, antimetabolites, mitotic inhibitors, antineoplastic antibiotics, hormonal agents, and other miscellaneous drugs.

Chemotherapy drugs are usually given in various combinations of dosages and timing, which are referred to as regimens or protocols. This offers many advantages, because when drugs with different targets or mechanisms of actions are combined, the chances for success are greater.

Chemotherapy may be used in addition to surgery and radiation. Sometimes chemotherapy is started before surgery in an attempt to decrease tumor size or stage of malignancy, thus improving the chances of successful surgery.

The decision to use chemotherapy depends on the type of tumor to be treated, the stage of malignancy, the condition of the animal, and financial constraints. Responses to cancer chemotherapy can range from a decrease in day-to-day signs but no overall increased survival time to a complete remission. Some tumors respond better to chemotherapy than others. For example, canine lymphoma is usually very responsive to chemotherapy. You should discuss the options and the risks and benefits of each with your veterinarian if you are considering chemotherapy to treat cancer in your pet.

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