Chlorambucil for Cats: Uses for IBD, Lymphoma & Dosage

This information is for educational purposes only. Never give your pet any medication without your veterinarian's guidance. Dosing, frequency, and safety depend on your pet's specific health profile.

Chlorambucil is an oral chemotherapy drug that vets also use as an immunosuppressive medication in cats. It belongs to a group called alkylating agents, which work by damaging DNA in rapidly dividing cells. In practice, that means it may help slow certain cancers and also calm an overactive immune response.

In cats, chlorambucil is most often prescribed off-label for small-cell gastrointestinal lymphoma and for some chronic inflammatory bowel disease cases that are not controlled well enough with diet changes, prednisolone, or other first-line options. Off-label use is common in veterinary medicine, but it also means your vet's instructions matter more than the human package insert.

This medication is considered hazardous to people handling it. Tablets should not be crushed, split, or compounded at home unless your vet and pharmacy specifically direct that plan. Many cats receive it as a tablet or a veterinary-compounded liquid, and your vet may recommend giving it with food to reduce stomach upset.

The two uses pet parents hear about most are feline IBD/chronic enteropathy and small-cell lymphoma. In chronic intestinal inflammation, chlorambucil may be added when a cat still has vomiting, diarrhea, weight loss, poor appetite, or low vitamin B12 despite diet trials and steroid therapy. In these cases, the goal is long-term control, not a quick fix.

For small-cell or low-grade gastrointestinal lymphoma, chlorambucil is commonly paired with prednisolone. This form of lymphoma often behaves more slowly than large-cell lymphoma, so an oral at-home protocol may be appropriate for some cats. Your vet may recommend ultrasound, intestinal biopsies, lab work, and cobalamin testing before deciding whether the problem looks more like IBD, lymphoma, or overlap between the two.

Chlorambucil can also be used for some other cancers or immune-mediated diseases, but those plans are more individualized. The right choice depends on your cat's diagnosis, blood counts, infection risk, liver and kidney values, and how practical home dosing and monitoring will be for your family.

There is no one-size-fits-all chlorambucil dose for cats. Your vet chooses the schedule based on the diagnosis, your cat's body size, bloodwork, and treatment goals. Published veterinary references list several common approaches. For inflammatory bowel disease or chronic colitis, Merck lists 2 mg/m2 by mouth every 48 hours, and another Merck reference describes 0.1-0.2 mg/kg or 2 mg per cat every 48-72 hours for 4-8 weeks or until signs improve. For small-cell lymphoma, many oncology protocols use intermittent dosing, often alongside prednisolone, but exact schedules vary by oncologist and case.

Because chlorambucil can suppress bone marrow, your vet will usually recommend baseline and follow-up bloodwork. CBC monitoring is especially important during the first weeks to months, then often every 1-3 months once the plan is stable. If blood cell counts drop, your vet may pause treatment, lower the dose, or lengthen the interval between doses.

Give chlorambucil exactly as prescribed. Do not double a missed dose unless your vet specifically tells you to. Tablets should not be crushed or split at home, and anyone handling the medication should wear disposable gloves. Pregnant or nursing people should avoid handling this drug. Your vet or pharmacy may also advise careful handling of litter box waste, vomit, and stool for a few days after each dose.

The most important risk with chlorambucil is bone marrow suppression, which means the body may make fewer white blood cells, red blood cells, or platelets. That can raise the risk of infection, tiredness, pale gums, bruising, or abnormal bleeding. Some cats feel fine even when counts are dropping, which is why scheduled lab monitoring matters so much.

Digestive upset is also possible. Pet parents may notice vomiting, diarrhea, decreased appetite, or lower energy. More serious warning signs include bloody diarrhea, shortness of breath, bruising, seizures, facial twitching, or marked lethargy. See your vet immediately if any of those happen.

Chlorambucil should be used cautiously or avoided in cats with active infection, existing bone marrow disease, pregnancy, or nursing. If your cat seems weaker, stops eating, develops a fever, or has any unusual bleeding, contact your vet promptly rather than waiting for the next scheduled recheck.

Chlorambucil has fewer day-to-day household interactions than some medications, but the important ones can be serious. VCA advises caution when it is combined with other myelosuppressive drugs, because stacking medications that lower blood cell counts can increase the chance of anemia, infection risk, or bleeding problems.

Vaccines are another key consideration. Because chlorambucil suppresses the immune system, your vet may delay certain vaccines or adjust the timing based on your cat's health status and exposure risk. This is especially relevant for cats being treated for lymphoma or chronic intestinal disease over many months.

Tell your vet about every medication and supplement your cat receives, including prednisolone, antibiotics, appetite stimulants, probiotics, herbal products, and compounded medications. That full list helps your vet decide what monitoring is needed and whether any part of the plan should be adjusted for safety.