Updated September 25, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
Quick answer: Chlorambucil (brand name Leukeran) is an oral chemotherapy drug that vets use mainly for low-grade (small cell) lymphoma in cats, some chronic leukemias, and certain immune or inflammatory diseases in cats and dogs. It’s usually given with prednisolone and is generally well tolerated. Its main risk is lowering blood cell counts, so regular bloodwork is essential. Because it’s a chemotherapy drug, it needs careful handling at home: wear gloves, never split or crush tablets, and take care with your pet’s urine and stool.
What it’s used for
- Low-grade (small cell) intestinal lymphoma in cats: the most common use. Many cats respond well and live for years.
- Chronic lymphocytic leukemia in dogs and cats
- Some immune-mediated and inflammatory diseases, such as severe inflammatory bowel disease or certain skin diseases, when other drugs aren’t enough or aren’t tolerated
- Some other cancers, sometimes as part of low-dose (metronomic) protocols
How it’s given
Chlorambucil comes as small tablets and is often reformulated by a compounding pharmacy into smaller doses. Schedules vary widely: daily, every other day, or a larger dose every few weeks. Follow your vet’s instructions exactly, and never adjust the dose yourself.
Side effects
- Low white blood cells or platelets: the most important risk, which raises the chance of infection or bleeding. Your vet will check blood counts regularly, especially early in treatment.
- Stomach upset: reduced appetite, vomiting, or diarrhea, usually mild.
- Muscle twitching or tremors (myoclonus): rare, reported mainly in cats. Call your vet if you see it.
- Hair coat changes: loss of whiskers or slow regrowth of shaved fur in some cats.
When to call your vet
Call promptly if your pet has a fever, lethargy, loss of appetite for more than a day, vomiting or diarrhea that doesn’t settle, unusual bruising or bleeding, blood in urine or stool, or twitching or seizures.
Safe handling at home
- Wear disposable gloves when giving tablets, and wash your hands afterward.
- Never split, crush, or open chlorambucil tablets or capsules.
- Pregnant or breastfeeding people and children shouldn’t handle the medication.
- Store it in its labeled container, away from food and out of reach of children and pets.
- Take care with waste for the period your vet advises (often 48–72 hours after a dose): wear gloves to scoop litter or pick up stool, bag it before disposal, and clean up accidents with gloves and disposable towels.
- Return unused medication to your vet or pharmacy rather than throwing it away.
Drug interactions and precautions
- Tell your vet about all other medications and supplements.
- Your vet may adjust or delay vaccinations during treatment.
- Pets with liver disease or low blood counts may need a different plan.
- Don’t use chlorambucil in pregnant animals.
Frequently asked questions
How long can a cat with small cell lymphoma live on chlorambucil?
Many cats with low-grade lymphoma treated with chlorambucil and prednisolone go into remission and live for 2 years or more. Your vet or veterinary oncologist can give you a more specific outlook.
Will my pet lose its hair?
Significant hair loss is uncommon in cats and most dogs, though cats may lose whiskers.
Is chemo painful for pets?
Veterinary chemotherapy aims to maintain quality of life, and most pets on chlorambucil feel well. Tell your vet about any side effects so they can adjust treatment.
Sources
- Kiselow MA, et al. Outcome of cats with low-grade lymphocytic lymphoma: 41 cases (1995–2005). JAVMA. 2008
- Merck Veterinary Manual: Antineoplastic agents
- Merck Veterinary Manual: Lymphoma in cats
- Plumb’s Veterinary Drug Handbook: Chlorambucil
- US National Institute for Occupational Safety and Health (NIOSH): List of hazardous drugs in healthcare settings
This article is for general education and is not a substitute for care from your own veterinarian. Learn about The Pet Vet and our editorial policy.