Cyclophosphamide for Dogs and Cats

Molly Price, DVM
By Molly Price, DVM on Dec. 19, 2023
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PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function, their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet.

What Is Cyclophosphamide for Dogs and Cats?

Cyclophosphamide is a prescription anti-cancer (chemotherapy) medication used to treat certain cancers in dogs and cats, including lymphoma, sarcoma, and carcinomas.

Cyclophosphamide can be administered as an oral medication or intravenously (directly into a vein under the skin) under the direct supervision of a veterinarian in the hospital. Cyclophosphamide is part of the CHOP protocol, where it is used in combination with other chemotherapy medications (doxorubicin, vincristine, and prednisone) to treat certain cancers.

Cyclophosphamide is FDA-approved for human use as generic oral and injectable formulations. The brand name Cytoxan® (in both oral and injectable formulations) has been discontinued by the manufacturer and is no longer commercially available. Cyclophosphamide is currently not FDA-approved as a veterinary medication. However, it is readily utilized in the veterinary field, and veterinarians can legally prescribe certain human drugs in animals in certain circumstances. This is called extra-label or off-label use because this use isn’t described on the drug label.

In certain circumstances, your vet may recommend a compounded formulation of cyclophosphamide. Compounded medications are prescribed if there’s a specific reason your pet’s health can’t be managed by an FDA-approved drug, such as if your pet has trouble taking pills in capsule form, the dosage strength is not commercially available, or the pet is allergic to an ingredient in the FDA-approved medication. Compounded medications are not FDA-approved. They are created by either a veterinarian or a licensed pharmacist on an individual basis to best suit a patient’s particular needs. You can learn more about compounded medications here.

Cyclophosphamide Considerations for Dogs and Cats

Due to the potential risk of toxicity to pets, veterinary staff, and pet parents, as well as the constantly evolving nature of chemotherapy treatment protocols, monitoring, and safety guidelines, it is best to consult with a board-certified veterinary oncologist about the use of cyclophosphamide in your pet.

If they determine cyclophosphamide to be an appropriate treatment option, they will also recommend an appropriate dose and schedule for your pet.

Treatment with cyclophosphamide requires intensive monitoring and close supervision by your veterinarian. Before, during, and in the weeks after your pet’s treatment, your veterinarian will perform blood tests to evaluate their immune system function as well as the effects of the drug on the liver and kidneys.

Fluid therapy and a dosage of a diuretic tablet such as furosemide are also typically prescribed to help your pet urinate. This helps lower the risk of bloody urine due to severe bladder toxicity (sterile hemorrhagic cystitis), which can be a serious side effect of cyclophosphamide.

Cyclophosphamide should not be used in pregnant or nursing pets, in pets with certain medical conditions, such as sterile hemorrhagic cystitis, kidney disease, liver disease, current infection, or bone marrow depression, or in pets who are hypersensitive or allergic to it.

Giving cyclophosphamide with certain medications can result in health risks to your pet. Always discuss with your veterinarian your pet’s current medications, including any vitamins and supplements they are taking, as well as medical conditions.

How Cyclophosphamide Works in Dogs and Cats

Cyclophosphamide belongs to a class of chemotherapy medications known as alkylating agents. Cyclophosphamide breaks down into an active compound that directly interferes with the DNA and RNA genetic replication processes that are essential to the survival of cancer cells, thereby preventing them from successfully dividing and replicating.

It is important to note that like many other chemotherapy drugs, cyclophosphamide affects unhealthy dividing cancer cells as well as healthy dividing cells throughout the body, such as those in the bone marrow. This lowers the bone marrow’s ability to produce blood cells, which results in low blood counts and increases your pet’s risk for infection.

Cyclophosphamide Directions for Dogs and Cats

Follow the directions on the drug label or as provided by your veterinarian. Your veterinarian may instruct you to give your pet’s cyclophosphamide dose in the morning.

Do not handle cyclophosphamide if you are pregnant, trying to become pregnant, or breastfeeding.

Wear gloves when handling cyclophosphamide.

Cyclophosphamide can be given with or without food, but giving it with food can decrease the risk of digestive upset.

Always provide your pet with plenty of water when they are taking cyclophosphamide.

Encourage your pet to drink plenty of water throughout the day, and take them on frequent walks to help empty the bladder as often as possible to lower the risk of bladder toxicity.

In the interest of public safety, do not take your pet to public places like parks where children will be playing or where there are other dogs, as your pet’s bodily waste (urine and stool) may contain traces of the medication.

Cyclophosphamide should be avoided in breeding, pregnant, and lactating pets, and should not be used in pets that are allergic or sensitive to it.

Missed a Dose?

The dosing schedule for cyclophosphamide is highly specific to your pet, their medical needs, and the type of cancer or condition being treated. Speak with your veterinarian right away about what to do if you forget to give a dose of cyclophosphamide. Do not give extra or double doses.

Cyclophosphamide Possible Side Effects in Dogs and Cats

Side effects of cyclophosphamide may include:

  • Bone marrow suppression—decreases in white blood cell, red blood cell, and platelet counts that may appear as infections, bleeding, or bruising

  • Gastrointestinal upset–loss of appetite (especially in cats), nausea, vomiting, and diarrhea

  • Blood in the urine due to bladder and kidney toxicity

  • Changes in hair coat, including shagginess and decreased luster

  • Hair loss (alopecia) in breeds with continuously growing haircoats such as Poodles, Afghan Hounds, and Old English Sheepdogs

Over the course of treatment, your veterinarian may need to delay or adjust your pet’s cyclophosphamide regimen, depending on your pet’s response and any side effects they experience due to cyclophosphamide.

Since cyclophosphamide can stay active in the body for several days, some side effects may persist, even after you have stopped administering the medication to your pet. Side effects may also last for a longer time in animals with liver or kidney disease.

Human Side Effects

Cyclophosphamide is also a prescription medication for humans, frequently with dosages different from those prescribed for your pet by a veterinarian. Due to possible side effects, humans should never use medicine dispensed for their pets and pets should not be given any medicine dispensed for a human’s use.

If you are pregnant, trying to become pregnant, or breast-feeding, talk to your veterinarian about the possibility of administering an alternative medication to your pet.

Cyclophosphamide is a chemotherapy drug, and it can be hazardous to other animals and people who come in contact with it. Your veterinarian will give you specific instructions including (but not limited to) the following precautions:

  • Ensure that you do not ingest cyclophosphamide.

  • Wear disposable gloves at all times while in contact with this medication and wash your hands after handling.

  • If any medicine contacts your skin, wash it off immediately and contact your physician.

  • Cyclophosphamide and its byproducts are found in a treated animal's secretions and bodily waste, including saliva, vomit, urine, stool, blood, and cat litter. Wear disposable gloves when cleaning up waste and handling the cat litter box.

  • Do not allow your pet to lick your skin, especially your hands and face.

  • Waste must be sealed in a plastic bag and disposed of according to your veterinarian’s instructions.

If you accidentally ingest this medication, immediately seek medical attention. If you come into contact with this medication without wearing gloves, immediately wash it off and call your physician or the national Poison Control Center hotline at 800-222-1222.

Call Your Vet If:

  • Severe side effects are seen (see above)
  • Your pet’s condition worsens or does not improve with treatment
  • You see or suspect an overdose
  • You have additional questions or concerns about the use of cyclophosphamide

Cyclophosphamide Overdose Information in Dogs and Cats

An overdose of cyclophosphamide can be serious. Signs of an overdose may include infection (associated with low white blood cell count from bone marrow suppression), loss of appetite, bloody urine, electrolyte imbalances, increased thirst, eyes darting back and forth (nystagmus), difficulty breathing, seizures, muscle tremors, and vomiting.

If you suspect an overdose, immediately contact your veterinarian, seek emergency veterinary care, or call an animal poison control center. Consultation fees often apply.

Pet Poison Helpline (855) 764-7661

ASPCA Animal Poison Control (888) 426-4435

Cyclophosphamide Storage for Dogs and Cats

Always confirm storage requirements by reading the prescription label. Compounded medications should be stored according to the compounding pharmacy’s label.

Cyclophosphamide capsules and tablets should be stored at room temperature from 68 to 77 F. Brief exposures to temperatures from  59 to 86 F are permitted.

Keep the container tightly closed in order to protect the medicine from moisture and light.

Keep out of reach of children and pets.

Cyclophosphamide for Dogs and Cats FAQs

What does cyclophosphamide do in dogs?

Cyclophosphamide is a prescription anticancer (chemotherapy) medication used in dogs to treat certain cancers including lymphomas, sarcomas, and carcinomas.

What are the side effects of cyclophosphamide in dogs?

Side effects of cyclophosphamide include infections, bleeding, or bruising due to low blood cell counts, gastrointestinal upset (loss of appetite, nausea, vomiting, diarrhea), blood in the urine, hair loss, and a dull shaggy hair coat.

How quickly does cyclophosphamide work in dogs?

Cyclophosphamide may take a few weeks before its full effects are noted. Effects of cyclophosphamide can vary from pet to pet and depend on a number of factors, such as your pet’s diagnosis and severity of the cancer.

No vet writer or qualified reviewer has received any compensation from the manufacturer of the medication as part of creating this article. All content contained in this article is sourced from public sources or the manufacturer.

Featured Image: AzmanJaka/E+ via Getty Images



Molly Price, DVM


Molly Price, DVM


Dr. Molly Price has practiced small animal medicine for over 20 years and is a graduate of Texas A&M College of Veterinary Medicine. She...

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