What Side Effects One May See after Chemotherapy Is Administered?

The goal of chemotherapy in veterinary medicine is to prolong good quality of life. Majority of pets undergoing chemotherapy do not develop side effects.

Side effects are seen when the normal, fast-growing cells are damaged significantly by chemotherapy. Here are some normal cells that grow quickly:

  • cells in the bone marrow (white blood cells, red blood cells and platelets) - this can make you pet feel tired, bruise and bleed easily, or put him/her at a higher risk of infection
  • cells in the stomach and intestines--this can cause your pet to have low appetite, vomiting or diarrhea
  • cells that grow hair--this can cause your pet to lose hair
The effect on these “innocent” cells is short lived and they recover usually within 1 week. Death occurs in less than 2% of patients receiving chemotherapy, when treatment for side effects is pursued by their owner.

Other rare side effects are related to administration of chemotherapy (tissue damage, allergic reactions or heart damage from long term use of certain chemotherapy drugs.

The bone marrow is found in the inner part of some bones. It is where all of the blood cells are made (red blood cells, white blood cells, and platelets). It is often affected by chemotherapy and can cause the bloodwork to be abnormal.

Red blood cells carry oxygen from the lungs to all parts of the body. During chemotherapy, the bone marrow may not be able to make enough red blood cells. Not having enough red blood cells is called anemia. It can make you pet feel weak, have low appetite, and low stamina. Anemia can also make the skin and gums look pale. The red blood cells are monitored closely during chemotherapy and their count is used to make treatment adjustments. If the anemia is severe, a blood transfusion may be indicated.

White blood cells fight infection. Chemotherapy lowers the white blood cell count (called neutropenia), which makes you pet less able to fight infections. This can develop within 5-10 days after chemotherapy. If a patient shows signs of an active infection (fever, acting sick, lethargy, collapse), hospitalization and treatment with intravenous medications is indicated. White blood cells are monitored closely during chemotherapy and their count is used to make adjustments in the drug doses or prescribe antibiotics.

Platelets are tiny pieces that form blood clots to stop bleeding and prevent the blood from “leaking” out of the vessels. If the bone marrow cannot make enough platelets, your pet may be at risk for spontaneous bleeding. This can cause bruises, nose bleeds, blood in urine, bleeding in the abdomen of chest or excessive bleeding from small cuts and be life threatening for your pet.. Fortunately, severe thrombocytopenia and active bleeding are rarely seen with the drugs and doses used in veterinary medicine.

Stomach and Intestinal cells can be damaged by chemotherapy, causing nausea, decreased appetite, vomiting, diarrhea. When seen, these problems develop within 2-7 days after chemotherapy. We are very proactive in prescribing medications to try to prevent or treat the nausea and intestinal discomfort. 

Hair Loss (Alopecia) - Most pets do not have continuous hair growth, like people and their coat is affected at a much lesser extent. Complete hair loss in pets is rare. Exceptions are certain breeds of dogs, such as Poodles, Old English Sheepdogs and other breeds whose hair grows continually. In general, if a pet needs to visit a groomer periodically to be clipped, then the pet may experience some degree of hair loss as a result of chemotherapy. The areas that are shaved for the administration of treatments or evaluations (ex. abdominal ultrasound) will regrow hair slower. Many pets (especially cats) lose all or most of their whiskers (this will not affected their quality of life). Hair and whiskers grow back after treatment, although they might be a different color or texture of hair.

Tissue Damage – Some chemotherapy drugs can cause severe tissues damage if accidentally given outside the vein. In our hospital chemotherapy agents are handled very careful and are only administered by highly trained technicians or doctors. We take a lot of safety precautions to decrease the risk of such accidents, including sedation, if necessary.

Allergic Reactions – This is a rare side effect seen during or shortly after giving certain chemotherapy drugs. Pets are generally pretreated with Benadryl prior to giving these chemotherapy drug, which greatly decreases the risk of seeing allergic reactions.  Also, they are kept for observation for 30-60 minutes after administration, which is the most common time when allergic reactions are seen.

Heart Damage - Some chemotherapy drugs (ie. doxorubicin= adriamycin) can irreversibly damage the heart muscle, especially when given at high doses. The dose used in veterinary protocols is below the dose that usually causes heart disease. Less than 5% of patients develop heart disease as a result of chemotherapy. A cardiac evaluation is recommended prior of using these drugs.

How Do We Avoid Side Effects from Chemotherapy?

The more chemotherapy we give, the more cancer and normal cells are affected. To decrease the side effects, we have to decrease the dose of chemotherapy but, in turn, this weakens the effects against the cancer. Therefore, we want to give the highest dose of chemotherapy that is well tolerated, to maximize the effects against the cancer, yet avoiding side effects.
As simple as it sounds, there is no simple way to find that perfect dose. Each individual and each cancer will have variations. No tests can be done to predict the side effects or the effect against the cancer cells. I start with a standard dose of chemotherapy that is well tolerated by most patients. If side effects are seen, I decrease the dose of chemotherapy in the future. If multiple adjustments are needed and the dose is significantly decreased, I may recommend switching to a different protocol or stopping chemotherapy.

In special circumstances, when there is a strong suspicion that the patient may not tolerate the treatment well (very sick patient, very small patients, etc) I may choose to start with a lower than standard dose initially and increase it in the future if it is well tolerated.

Each chemotherapy drug has its own risk of side effects. Just because a patient had a problem with a drug, doesn’t mean he/she will have problems with other drugs and vice-versa. Occasionally there can be a cumulative effect of giving chemotherapy long term that increases the risk of side effects as we give more chemotherapy. We monitor the patients closely for side effects and recommend modifications as needed.

How Is Chemotherapy Given?

Chemotherapy can be given as a pill, injection under the skin (subcutaneously), injection in the vein (intravenously), short infusion in the vein (intravenously), injection inside a body cavity (chest or abdomen), or directly inside the tumor.

The method of administration of chemotherapy treatments along with the dose is determined by rigorous testing called clinical trials, which are done prior to the specific chemotherapy drug being available for commercial use. During this testing process, scientists and doctors determine how chemotherapy drugs are absorbed in the body and how they work. Sometimes stomach juices can destroy the chemotherapy and such drugs must be administered as injections. Just because a chemotherapy drug comes in a pill form doesn’t mean it is a weaker drug.

What Is the Quality of Life of a Pet During Chemotherapy?

The goal of chemotherapy in veterinary medicine is to treat the cancer as aggressively as possible but without sacrificing the quality of life.  My goal is to improve and maintain good quality of life in my patients.

Along the years I treated patients for which maintaining excellent quality of life during chemotherapy was an absolute must: the assistance dog that continued to serve his wheelchair-bound owner for 3 months until he could be replaced; the professional hunting dog that continued to enjoy hunting, the competitive Cocker Spaniel that won the most important agility trial of his life; the Golden Retriever that continued to be a therapy dog in the pediatric oncology ward. These are great examples of what quality of life should be during chemotherapy.

The cancer itself also influences quality of life. If the treatment doesn’t control the cancer, the patient may develop poor quality of life due to the cancer effects on the body. If we cannot improve quality of life, it is better that we stop.

When Is Chemotherapy Used?

Chemotherapy goes everywhere in the body and it is used as a systemic treatment. This means the drugs travel throughout the body to reach cancer cells wherever they may have spread. It differs from surgery or radiation, which are considered local treatments, since they only act on a specific area.


Chemotherapy can also be used to prevent regrowth of a tumor that was not removed completely or to increase the potency of radiation therapy.

How Does Chemotherapy Work?

Chemotherapy has the greatest effect against fast growing cells and destroys cancer cells by keeping the cells from further multiplying.

Some cancer cells grow slowly while others grow rapidly. As a result, different types of chemotherapy drugs target the growth patterns of specific types of cancer cells. Each drug has a different way of working and is effective at a specific time in the life cycle of the cancer cells. Your doctor will determine the chemotherapy drug that is right for your pet.

Chemotherapy cannot tell the difference between a cancer cell and a normal cell. It is not specific and normal, healthy cells can also be affected, especially those that naturally multiply quickly. There is a direct relationship between the amount of chemotherapy given and the number of cells that get hurt. The more chemotherapy we give, the more cancer and normal cells are affected. With high doses of chemotherapy we can control the cancer better but we also see more side effects



Dr. Sarbu, our cancer specialist, works with clients from various locations throughout the Puget Sound, including (but not limited to) Renton, Bellevue, Redmond, Kirkland, Bothell, Lynnwood, Edmonds, Shorline, Seattle, Burien, Auburn, Kent, Federal Way, Maple Valley, Tacoma and Puyallup.