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Cyclophosphamide, commonly known as Cytoxan, is an immunosuppressant that treats several types of cancer and a range of immune-mediated diseases. This powerful drug is an alkylating agent that kills rapidly growing cells, including cancer cells.

Cytoxan is commonly used alongside other medications to treat canine lymphomas, leukemias, carcinomas, and sarcomas. Keep reading to find out more about the dosage, efficacy, and side effects of Cytoxan for dogs.

Dosage

Cyclophosphamide can be given orally in a tablet form (25 mg and 50 mg tablets available) or via an intravenous injection.

When treating cancer, Cytoxan is used in conjunction with other medications as part of a metronomic chemotherapy program. Dosage amounts vary based on the size of the dog and the condition, so make sure to follow dosage instructions provided by your veterinarian closely.

Efficacy

There’s an abundance of scientific evidence regarding the efficacy of cyclophosphamide in treating cancer. It inhibits tumor angiogenesis and suppresses regulatory T cells (Treg) in mice and humans. Studies indicate that it can have similar effects on dogs.

Cyclophosphamide is often part of an effective long-term chemotherapy strategy. Studies show that metronomic chemotherapy with cyclophosphamide and piroxicam prevents tumor recurrence in dogs with soft tissue sarcomas. It’s also an important component in the majority of metronomic chemotherapy regimens.

However, cyclophosphamide chemotherapy can cause sterile hemorrhagic cystitis (SHC) in up to 23% of cases, so close monitoring and/or prophylactic treatments are recommended for dogs undergoing chronic metronomic cyclophosphamide therapy. Cyclophosphamide can also cause bone marrow suppression and gastrointestinal toxicity in some dogs.

Cyclophosphamide can also be used to treat some immune system disorders, including inflammatory bowel disease, but usually only after other treatments have failed.

Side effects

Cytoxan for dogs can produce a wide range of side effects. These may include:

  • Nausea

  • Loss of appetite

  • Gastrointestinal upset (including vomiting and diarrhea)

  • Hemorrhagic cystitis (which can lead to blood in urine)

  • Bone marrow suppression

  • Hair loss or changes in hair texture

Another potential side effect of Cytoxan for dogs is the suppression of white blood cell counts. This may lead to infections, bleeding, and bruising, so many veterinary oncologists like to monitor white blood cell count during treatment.

Make sure you report any symptoms or side effects of cyclophosphamide treatment to your veterinarian.

Considerations

As with any other medication, there are certain risks associated with giving Cytoxan for dogs. It’s also essential that you inform your veterinarian of any other medications your pet may be taken before Cytoxan is prescribed.

Drug interactions

Cytoxan may interact with the following drugs:

  • Gout medications (allopurinol)

  • Antibiotics (chloramphenicol

  • Immunosuppressants (cyclosporine)

  • Heart medications (digoxin)

  • Hyperthyroidism medications (methimazole) 

  • Antiemetics used to treat nausea and vomiting (ondansetron)

  • Epilepsy medications (phenobarbital)

  • Thiazide diuretics

Make sure your veterinarian is aware of all medications your dog is currently taking.

Allergic reactions and sensitivity

You should never give cyclophosphamide to a pet known to be allergic to the drug. And because it suppresses the immune system, Cytoxan should not be used in dogs with serious infections.

It is also toxic to the cells of a developing fetus, so should only ever be administered to a pregnant dog when the benefits outweigh the significant risks.

Finally, caution should be used when giving Cytoxan to dogs with kidney disease, liver disease, or bone marrow problems. 

Frequently asked questions

What happens if I forget to give a dose of Cytoxan to my dog?

You should never give a double dose of cyclophosphamide to your pet. Instead, ask your veterinarian for instructions on how to continue administering the drug to your dog.

Can I get Cytoxan for dogs without a prescription?

No. Cytoxan is only available with a prescription from a veterinarian. 

Do I need to take any precautions when administering Cytoxan to my dog?

Yes. Cyclophosphamide can cause immune suppression in humans, so be sure to wear disposable gloves when handling this medication and wash your hands immediately afterward. You’ll also need to wear disposable gloves when disposing of your pet’s waste for a few days after they’ve taken Cytoxan. You should never handle this drug if you’re pregnant or breastfeeding. Finally, never flush any leftover tablets down the toilet.

Can I split or cut Cytoxan tablets in half?

Not without your veterinarian’s guidance. This makes it difficult to provide an accurate dosage to your pet. 

How should I store Cytoxan?

Store Cytoxan tablets at room temperature and away from light and moisture.

What should I do if my pet has an overdose of Cytoxan?

Seek veterinary attention for your pet immediately.

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