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Voriconazole

By Rania Gollakner, BS, DVM, MPH

Medications

What is voriconazole?

Voriconazole (brand name: Vfend®) is an antifungal medication used to treat fungal infections, such as blastomycosis, cryptococcosis and aspergillosis. Its use in veterinary medicine is new and information is currently limited.

Its use in dogs, cats, horses, birds, and reptiles to treat fungal infections 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 voriconazole given?

Voriconazole is given by mouth in the form of a tablet or liquid. Give at least 1 hour before or 1 hour after feeding. If your pet vomits after receiving the medication on an empty stomach, give future doses after a meal. Shake the liquid form for at least 10 seconds prior to dosing, and measure the liquid carefully. It can also be given as an injection in the hospital setting.

This medication can take a few weeks before full effects are noted, but gradual improvements are usually noticeable after a few days.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give it at the next scheduled time, and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

Side effects are not thoroughly known in animals, but may include vomiting, diarrhea, or decreased appetite. In dogs, liver enlargement has occurred after 30 days of dosing. In cats, noted side effects include kidney disease, lack of appetite, tiredness, skin reactions, incoordination, limb paralysis, eye/vision problems, abnormal heart rhythms, and drooling. Serious side effects may include skin rash, yellow skin or eyes, difficulty walking or paralysis, persistent lack of appetite, or vision problems. Limited studies have been completed in animals, but in humans, commonly reported effects include vision problems, skin reactions, and less commonly gastrointestinal effects, liver problems, cardiovascular effects, and electrolyte imbalances.

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 give voriconazole in pets that are allergic to it or other azole antifungals. Use with extreme caution in cats as the side effects can be serious and dangerous. Use cautiously in pets with liver disease, kidney disease, or abnormal heart rhythms. Do not use in pregnant pets unless it is necessary. Use cautiously in lactating pets and consider a milk replacer as a precaution.

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with voriconazole: antidiabetic agents, barbiturates, benzodiazepines, calcium-channel blockers, carbamazepine, cisapride, corticosteroids, cyclosporine, fentanyl, immunosuppressive agents, methadone, phenytoin, proton-pump inhibitors, quinidine, rifampin, rifabutin, vinca alkaloids, or warfarin. 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?

Liver enzymes and serum electrolyte levels will be tested regularly while on this medication. Your veterinarian may monitor your pet to be sure that the medication is working. Because the side effects in animals are not well known yet, monitor for any possible problems such as skin rash, yellow skin or eyes, difficulty walking, appetite abnormalities, or vision problems.

How do I store voriconazole?

Store the tablets and the liquid at room temperature between 59°F and 86°F (15°C and 30°C). Store the powder used to make the oral liquid in the refrigerator between 36°F and 46°F (2°C and 8°C); once mixed, the liquid form should be discarded after 14 days.

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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