Flucytosine

What is flucytosine?

Flucytosine is an antifungal drug used in combination with other drugs to treat serious fungal infections in pets, including Cryptococcus spp., Candida spp., and Aspergillus spp. If flucytosine is used alone, pets can rapidly develop resistance to it.

No veterinary products are available. Human products (e.g., Ancobon®) are used off label in dogs, cats and birds, although caution is advised for use in dogs as they may develop severe skin reactions.

Many drugs are commonly prescribed for off-label use in veterinary medicine. “Off label” means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. 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 do I give my pet flucytosine?

• Flucytosine is given orally (by mouth), in the form of a capsule.
• Flucytosine may be given with or without food.
• If vomiting occurs when dosed on an empty stomach, give future doses with food or a treat.
• Follow the instructions on the label of your product, in conjunction with your veterinarian’s directions.
• This medication must be given in conjunction with another antifungal drug.
• If you are pregnant or nursing, avoid handling flucytosine or wear gloves when handling it. Flucytosine has caused birth defects in laboratory animals.

This short-acting medication should take effect within 1–2 hours, and improvement in clinical signs should follow.

What if I miss giving my pet the medication?

If you miss a dose of flucytosine, give it as soon as you remember, and then continue with the regular dosing schedule. However, if it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once.

Are there any potential side effects from flucytosine?

Flucytosine may cause the following side effects:

• Nausea, vomiting, diarrhea
• Oral ulceration

Serious adverse reactions associated with this drug may occur without warning and could, in rare cases, lead to death. If any of the following signs develop, stop giving flucytosine and contact your veterinarian immediately:

• Severe skin lesions such as ulcers, crusts and weeping wounds
• Seizures
• Unusual behavior
• Changes in blood cell numbers (low red blood cell, white blood cell, and platelet counts), identified through blood work

Drug sensitivities can develop over time with repeated exposure. Your pet may have no reaction after the first few doses but may develop a sensitivity later. It is important to watch for signs of an allergic reaction over the entire course of treatment.

Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately. This short-acting medication should stop working within 12 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

DO NOT USE flucytosine in:

• Pets with a known hypersensitivity to this medication
• Pets with bone marrow suppression

USE WITH EXTREME CAUTION in:

• Dogs (potential for serious side effects on the skin)
• Pets with kidney impairment

Are there any drug interactions I should be aware of?

Certain medications may interact with flucytosine, so be sure to tell your veterinarian about any medications (including vitamins, supplements, and herbal therapies) that your pet is taking before starting any new treatment.

Several medications should be used with caution when given with flucytosine, including:

• Aluminum- or magnesium-containing antacids
• Cytarabine
• Myelosuppressive drugs (e.g., antineoplastics, azathioprine, chloramphenicol, methimazole)
• Zidovudine
• Amphotericin B

Note: Amphotericin B is given in combination with flucytosine to increase the effectiveness of treatment. However, when using these medications together, it is vital to pay close attention to kidney (renal) function, and adjust dosages if renal toxicity develops.

Is any monitoring needed with this medication?

• Blood work is required to assess a complete blood count (red cells and white cells, with platelets).
• Blood work to assess liver function should be performed monthly.
• Blood work to assess kidney function should be performed at least twice weekly if also receiving amphotericin B.
• Flucytosine serum levels should be monitored, if possible.
• Your veterinarian will let you know when they want to reassess your pet to ensure the medication is working and not causing side effects.

At home, monitor for any side effects such as those noted above. If any side effects occur, discontinue use of the medication and contact your veterinarian.

How do I store flucytosine?

• Store at 25°C (77°F); excursions permitted to 15°C–30°C (59°F–86°F).
• Store in an airtight container.
• Protect from light.
• Keep medication out of reach of children and pets.

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. Be prepared to provide information about the product and your pet’s weight and symptoms.