By Rania Gollakner, BS, DVM, MPH

What is hydroxyurea?

Hydroxyurea (brand names: Hydrea®, Droxia®, Mylocel®, Dacrodil®, Droxiurea®, Hydrine®, Litalir®, Medroxyurea®, Neodrea®, Onco-Carbide®, Oxeron®, Syrea®, Siklos®) is an antineoplastic medication used to treat certain types of cancers, such as polycythemia vera, mast cell tumors, meningioma, and leukemia. It may also be useful in treating feline hypereosinophilic syndrome and canine meningiomas.

Its use in cats and dogs 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 hydroxyurea given?

Hydroxyurea is given by mouth in the form of a capsule. Give with food to help limit side effects such as vomiting and diarrhea. A compounded liquid form may also be available; shake well before dosing and measure liquid forms carefully.

Hydroxyurea is considered a hazardous drug. Do not handle if you are pregnant or nursing. Wear gloves when handling this medication and when handling all bodily waste (urine, feces, blood, vomit, etc.) for several days after dosing this medication. Place gloves and contaminated waste in a sealable bag before disposing in the regular garbage. If skin or eye contact occurs while dosing this medication, wash carefully with soap and water or rinse eyes thoroughly.

Pregnant women should not handle this medication or waste from the treated pet.

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?

Because this medication must be dosed very carefully to avoid toxicity, please contact your veterinarian for further instructions if you miss a dose.

Are there any potential side effects?

The most common side effect is bone marrow suppression which causes low blood cell counts; this side effect should be closely monitored by your veterinarian. Other side effects include lack of appetite, vomiting, diarrhea, mouth and stomach ulcers, loss of toenails and/or hair coat, and urinary problems.

Serious side effects include bleeding, bruising, fever, lethargy, coughing, or difficulty breathing due to lung damage; contact your veterinarian immediately if these side effects occur.

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?

Do not use hydroxyurea in pets that are allergic to it. It should be used cautiously in pets with anemia, bone marrow problems, active infection, lung disease, urate urinary stones, kidney disease, or pets with a history of chemotherapy or radiotherapy. Hydroxyurea should not be used in pregnant pets unless absolutely necessary. Do not use hydroxyurea in lactating pets; use a milk replacer if hydroxyurea is used.

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with hydroxyurea: myelosuppressive agents or vaccines. Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

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

Your veterinarian will perform bloodwork (complete blood count), including the number of platelets at least every 1-2 weeks. Once stable, bloodwork should be monitored every 3 months. Kidney values should also be measured before the start of treatment and then every 3-4 months. Your veterinarian may monitor your pet to be sure that the medication is working.

Monitor your pet for bleeding, bruising, fever, difficulty breathing, or severe lethargy, as this could indicate toxicity. If you note any of these signs, contact your veterinarian as this is an emergency.

How do I store hydroxyurea?

Store capsules at room temperature between 68°F and 77°F (20°C and 25°C). Store away from light and in a tightly closed container. Store the compounded liquid at room temperature for up to 14 days, unless otherwise directed on the label.

What should I do in case of 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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