Lomustine for Dogs

Updated Nov. 21, 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 Lomustine?

Lomustine is a prescription anti-cancer (chemotherapy) medication used to treat certain cancers in dogs, and less frequently in cats.

Lomustine is used for the treatment of cancers involving the brain and neurological system, mast cell tumors, relapsed lymphoma in dogs, histiocytic sarcoma in dogs, and fibrosarcoma in cats.

Lomustine can be administered as an oral medication in combination with other chemotherapy medications or  intravenously (under the skin) under the direct supervision of a veterinarian in the hospital.

Lomustine is currently not an FDA-approved brand for veterinary medication. However, it is FDA-approved for human use under the brand name Gleostine®. The generic form of lomustine and the brand name CeeNU® (abbreviated as CCNU) are no longer available. 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 lomustine. 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.

Lomustine Considerations

Because of the potential risk of toxicity to pets, veterinary staff, and pet parents, and since chemotherapy treatment protocols, monitoring, and safety guidelines often change, it is best to consult with a board-certified veterinary oncologist about the use of lomustine in your pet.

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

Treatment with lomustine 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. A supplement for liver support and repair such as Nutramax® Denamarin is often prescribed to help reduce the risk of liver toxicity in dogs receiving lomustine.

Lomustine should not be used in pregnant or nursing pets, in pets with certain medical conditions, such as liver disease, kidney disease, current infection, bone marrow depression, lung problems, in pets who have recently been vaccinated or are scheduled for a vaccination, or in pets who are hypersensitive or allergic to it.

Giving lomustine 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 Lomustine Works

Lomustine is classified as a chemotherapy medication that works as an alkylating agent. Alkylating agents directly interfere with the genetic replication process of DNA, RNA, and crucial proteins that are essential to the survival of cancer cells, thereby preventing them from successfully dividing and replicating.

Lomustine’s unique lipophilic (fat-dissolving) property allows it to move from the bloodstream into the brain and central nervous system to target cancer cells there.

It’s important to note that like many other chemotherapy drugs, lomustine affects unhealthy dividing cancer cells as well as healthy dividing cells such as those in the bone marrow. Lomustine decreases the bone marrow’s ability to produce blood cells, resulting in low blood counts that may increase your pet’s risk for infection.

Lomustine Directions

Follow the directions on the drug label or as provided by your veterinarian.

Do not handle lomustine if you are pregnant or trying to become pregnant.

Wear gloves when handling lomustine.

Ask your veterinarian if you should give lomustine to your pet on an empty stomach.

Lomustine should not be used in pets that are allergic or sensitive to it.

Lomustine should be avoided in pregnant and lactating pets.

Missed a Dose?

The dosing schedule for lomustine is strict and 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 lomustine. Do not give extra or double doses.

Lomustine Possible Side Effects

Side effects from lomustine may include:

  • Gastrointestinal upset (low appetite, vomiting, diarrhea)

  • Bleeding or bruising

  • Fever (indicating infection)

  • Yellowing of the whites of the eyes, skin, or gums (signs of jaundice associated with liver toxicity)

  • Hair loss

  • Low energy (lethargy)

  • Weakness

  • Trouble breathing or coughing

  • Sores in the mouth and gums

  • Eye redness, irritation, discharge, or discomfort

Your veterinarian may need to delay your pet’s treatment or adjust the dose of lomustine, depending on your pet’s response and any side effects that they experience.

Human Side Effects

Lomustine 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.

Lomustine 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 lomustine.

  • 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.

  • Lomustine and its byproducts are found in treated animal's secretions and bodily waste (saliva, vomit, urine, stool, blood, and litter). Wear disposable gloves when cleaning up waste and handling the cat litterbox.

  • 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)

  • You see or suspect an overdose

  • You have additional questions or concerns about the use of lomustine

Lomustine Overdose Information

Lomustine has a narrow margin of safety, meaning that even a small amount given over the prescribed dose can result in poisoning or toxicity.

An overdose of lomustine can be serious. The severity of a lomustine overdose depends on the species, the amount given by weight, and for how long it was given.

Signs of an overdose may include low energy, weakness, bleeding or bruising, fever, coughing or trouble breathing, yellowing of the eyes, skin, and gums, and gastrointestinal upset (loss of appetite, vomiting, diarrhea).

If you suspect an overdose, immediately contact your veterinarian to 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

Lomustine Storage

Lomustine should be stored at controlled temperatures from 68 to 77 F. Brief exposures to temperatures from 59 to 86 F are permitted. Avoid exposure to temperatures above 104 F.

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

Always confirm storage requirements by reading the prescription label.

Compounded medications should be stored according to the compounding pharmacy’s label.

Keep out of reach of children and pets.

Lomustine for Dogs FAQS

How much does lomustine cost for dogs?

The cost for lomustine can vary depending on the dosage and frequency schedule your veterinarian recommends, your dog’s weight, the type and stage of cancer being treated, their health status, and their individual needs. Your veterinarian will adjust the dosage and frequency throughout the treatment.

What are the side effects of lomustine in dogs?

Side effects from lomustine may include low energy (lethargy), weakness, bleeding or bruising, fever (indicating infection), trouble breathing or coughing, yellowing of the whites of the eyes, skin, or gums (jaundice associated with liver toxicity), gastrointestinal upset (low appetite, vomiting, diarrhea), sores in the mouth and gums, hair loss, and redness, irritation, discharge, or discomfort of the eyes.

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: SeventyFour/iStock via Getty Images Plus

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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