Voriconazole

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 (extra label). 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 carefully, as they 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. It can also be given as an injection in the hospital setting or compounded into an eye drop.
• Shake the liquid form for at least 10 seconds prior to dosing, and measure the liquid carefully. 
• Give voriconazole at least 1 hour before feeding or 1 hour after feeding. 
• If your pet vomits after receiving the medication on an empty stomach, give future doses after a meal. 

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 as soon as you remember, and then continue with the regular dosing schedule. 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?

Side effects of voriconazole are not thoroughly known in animals, but may include:

• Vomiting, diarrhea, decreased appetite
• Liver enlargement (in dogs, after 30 days of dosing)

Voriconazole may cause the following side effects in cats:

• Kidney disease
• Lack of appetite
• Tiredness
• Skin reactions
• Incoordination
• Limb paralysis
• Eye/vision problems
• Abnormal heart rhythms
• Drooling

Serious adverse reactions may occur. If any of the following signs develop, stop giving voriconazole and contact your veterinarian immediately:

• Rash
• Yellow skin or eyes
• Difficulty walking or paralysis
• Persistent lack of appetite
• Vision problems

Limited studies have been completed in animals, but in humans, reported effects include:

• Vision problems
• Skin reactions
• Gastrointestinal effects
• Liver problems
• Cardiovascular effects
• Electrolyte imbalances

Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately. 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 voriconazole in pets that:

• Are allergic to it or other azole antifungals
• Are pregnant (unless necessary)

USE WITH EXTREME CAUTION in:

• Cats (side effects can be serious and dangerous)

USE WITH CAUTION in pets that:

• Have liver disease or kidney disease
• Have abnormal heart rhythms. 
• Are lactating (consider a milk replacer)

Are there any drug interactions I should be aware of?

Several medications should be used with caution when given with voriconazole: 

• Antidiabetic agents
• Barbiturates
• Benzodiazepines
• Calcium-channel blockers
• Cisapride
• Corticosteroids
• Cyclosporine
• Immunosuppressive agents
• Proton-pump inhibitors

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is any monitoring needed with this medication?

• Your veterinarian will test liver enzymes and serum electrolyte levels regularly while on this medication. 
• Your veterinarian will let you know when and how they want to reassess your pet to ensure the medication is working and not causing side effects. 
• 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 or as 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. Be prepared to provide information about the product and your pet's weight and symptoms.