By Rania Gollakner, BS, DVM, MPH

What is cyclophosphamide?

Cyclophosphamide (brand names: Cytoxan®, Neosar®, Procytox®) is an antineoplastic, anticancer medication used in combination with other medications to treat lymphomas, leukemias, carcinomas, and sarcomas. It was previously also used to treat immune-mediated diseases, but newer medications with fewer side effects are now available and are preferred in these cases.

Its use in cats, dogs, horses, and rabbits to treat certain cancers is ‘off label’ or ‘extra-label’. Many drugs are commonly prescribed for off label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their directions may be significantly different from those on the label.

How is cyclophosphamide given?

Cyclophosphamide is given by mouth in the form of a capsule or a specially compounded liquid. It can also be given as an injection in the hospital setting. It should be given with food to minimize stomach upset. Measure liquid forms extremely carefully. After dosing, encourage your pet to drink water and frequently urinate to decrease the risk of bladder toxicity.

Always wear gloves when giving this medication, and do not reuse gloves. Do not allow the medication to come into contact with your skin, eyes, or mouth. On the day of giving this medication and for a few days afterward, handle your pet’s bodily fluids carefully and wear gloves; seal the gloves and any waste in a plastic bag before disposing in the garbage. Do not allow your pet to lick your skin; if this occurs, wash the area thoroughly with soap and water. Pregnant women should not handle this medication.

This medication can take a few weeks before full effects are noted, but side effects can occur immediately. Gradual improvements are usually noticeable after a few days.

What if I miss giving my pet the medication?

If you miss a dose, contact your veterinarian for further instructions, as this medication must be dosed very carefully.

Are there any potential side effects?

Side effects include hair loss or changes in hair coat. More serious side effects include decreases in blood cell counts that may appear as infections, bleeding, or bruising, lack of appetite, vomiting, and diarrhea, blood in the urine, depression, lung damage, difficulty breathing, dizziness, seizures, or tremors.

This moderate-acting medication should stop working in a few days, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Cyclophosphamide should not be used in pets that are allergic to it or in pets with urinary obstruction, infection, or bone marrow dysfunction. It should be used cautiously in pets with liver or kidney disease, pets receiving radiation, or pets susceptible to infections (such as diabetics or geriatric pets). Discontinue use in pets that have had an episode of blood in the urine that is not associated with a bladder infection.

Cyclophosphamide should not be used if you cannot take your pet to your veterinarian for regular monitoring during treatment. It should not be used in pregnant animals unless it is required as a last resort.

Do not use in lactating animals unless necessary, and in these cases a milk replacer should be used.

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with cyclophosphamide: allopurinol, cardiotoxic drugs, immunosuppressants, myelosuppressive agents, thiazide diuretics, ondansetron, phenytoin, phenobarbital, vaccines (live), or warfarin.

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

This medication may also affect certain laboratory tests such as uric acid and antigenic skin tests.

Is there any monitoring that needs to be done with this medication?

Regular blood cell counts, kidney function tests and urine tests are required to monitor for toxicity. Your veterinarian may monitor your pet to be sure that the medication is working. Monitor closely for serious side effects and contact your veterinarian immediately if any occur.

How do I store cyclophosphamide?

Store the capsules of this medication between 68°F and 77°F (20°C and 25°C) with brief excursions between 59°F and 86°F (15°C and 30°C) allowed. Protect from light. Store liquid forms in the refrigerator in a glass container for up to 14 days.

What should I do in case of emergency?

Any overdose, no matter how small, should be considered an emergency. If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

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