Finding out your pet has cancer can be a frightening and stressful experience for many reasons. One common source of anxiety is the idea that your pet may require chemotherapy treatment, which can be an intimidating prospect. The word “chemotherapy” often conjures images of severe side effects that nobody wants their pet to experience: things like incessant vomiting, hospitalization, and total hair loss. The good news is that chemotherapy in veterinary patients looks different than it does in humans and can still result in a good quality of life during treatment. 

The concepts and methodology surrounding chemotherapy are unknown to most pet owners, making demystifying the mechanisms of these drugs important in becoming more comfortable pursuing treatments. To truly understand chemotherapy, it is necessary to step back and first discuss the normal behavior of the cell. In health, our cells operate under strict orders from a complicated series of chemical signals: cells are told when to divide, when to grow, and even when to die. Most cells in our body spend the majority of their time in a “quiescent” (quiet) phase, where they are not undergoing cell growth or division (mitosis) but are focusing on doing their physiologic jobs. Different cells in our body divide at different speeds depending on their role. For instance, once your heart is full-grown, those cells rarely need to divide. In contrast, the stomach lining is subject to the wear and tear of stomach acid and digestive enzymes, so these cells have a finite lifespan and are continuously regenerating. 

Unlike the well-regulated cells of healthy organs, cancer cells operate lawlessly. Their goal is to spread themselves widely, so they spend most of their time growing and dividing, ignoring the body’s typical chemical “stop” signals. Most chemotherapy drugs work by exploiting this feature of cancer cells, specifically targeting the cell division machinery. By doing so, chemotherapy is selectively toxic to cancer cells. However, because there are other rapidly dividing tissues in the body, chemotherapy does carry risk of (usually temporary) collateral damage.  

The organ systems most affected by chemotherapy include the stomach/intestinal tract (leading to a possibility for nausea and diarrhea) and the bone marrow, where the body’s blood cells are formed (leading to a possibility for immune system suppression). In the majority of patients, these side effects are not debilitating and can be managed at home with things like anti-nausea medication, dietary change, probiotics, or other supportive medication. The goal of veterinary chemotherapy is to allow our pets to have longer and happier lives while we manage their cancer. 

Chemotherapy does carry the risk of more serious side effects, though these are thankfully less common. Your veterinary oncologist will discuss the specifics of your pet’s case and strategize ways to mitigate risks, balancing efficacy with quality of life.