Voriconazole
UK-109496 (Synthetic derivative of fluconazole)
Also known as: Vfend · UK-109496 · voriconazol · voraconazolum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Coccidioidomycosis | 4 mg/kg PO q12h. | PO | q12h | — | 🌎 NA |
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Orbital aspergillosis | 10 mg/kg PO once daily. | PO | q24h | — | 🌎 NA |
- Orbital aspergillosis: Use with extreme caution; significant systemic and neurologic adverse reactions are common.
Birds
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General / Pharmacokinetic study | 18 mg/kg PO q8h for 11 days | PO | q8h | 11 days | 🌎 NA |
| Avian aspergillosis | 12.5 mg/kg PO twice daily for 60-90 days or by nebulization as a 1 mg/mL solution for 60 minutes once daily. | PO / Nebulization | q12h / q24h | 60-90 days | 🌎 NA |
- General / Pharmacokinetic study: Studied in Hispaniolan Amazon parrots; further studies needed for long-term safety.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Aspergillosis | 3 mg/kg PO q24h | PO | q24h | — | 🌎 NA |
- Aspergillosis: Higher doses (4-5 mg/kg) are probably necessary to treat infections with Fusarium spp.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Voriconazole is a broad-spectrum, second-generation triazole antifungal agent derived from fluconazole. It was developed to treat severe, refractory fungal infections.
- Broad Spectrum: Highly effective against a variety of fungal organisms, particularly Aspergillus, Blastomyces, and Cryptococcus, as well as Candida, Histoplasma, and Fusarium species.
- Clinical Use in Veterinary Medicine: Currently, there is limited clinical experience in veterinary patients. It is extremely expensive, which limits its widespread use, though it is gaining interest for treating aspergillosis in pet birds due to their smaller size.
- Tissue Penetration: It has excellent oral bioavailability in many species and readily crosses into the central nervous system (CNS), tears, and synovial fluid, making it valuable for systemic and deep-seated infections.
- Feline Sensitivity: Cats appear highly susceptible to severe adverse effects (neurologic and systemic). It should only be used in cats as an absolute last resort.
- Drug Interactions: Like other azoles, it is a potent inhibitor of cytochrome P450 enzymes, leading to numerous significant drug interactions.
Mechanism of action
Voriconazole exerts its fungistatic (and sometimes fungicidal against Aspergillus) effects by inhibiting fungal cell membrane synthesis.
- Primary Mechanism: It inhibits the cytochrome P-450-dependent enzyme 14-alpha-sterol demethylase (lanosterol 14-alpha-demethylase) → blocks the conversion of lanosterol to ergosterol → depletes ergosterol in the fungal cell wall → alters cell membrane fluidity and permeability, leading to fungal cell death.
- Secondary Mechanism: Unlike fluconazole, voriconazole also inhibits 24-methylene dehydrolanosterol demethylation in molds such as Aspergillus, which confers its enhanced activity against these specific fungi.
Safety & warnings
Contraindications
- Hypersensitivity to voriconazole or other azole antifungals
- Concurrent use with barbiturates, carbamazepine, cisapride, pimozide, quinidine, rifampin, or rifabutin
- Caution in patients with hepatic dysfunction
- Caution in patients with proarrhythmic conditions
- Caution with the IV formulation in patients with decreased renal function (due to accumulation of the SBECD vehicle)
Adverse effects
- Dogs: Liver enlargement, significant increase in cytochrome P450 hepatic enzymes
- Cats: Azotemia, inappetence, lethargy, weight loss, cutaneous drug reactions, ataxia, hind limb paresis/paraplegia, visual signs (mydriasis, decreased PLR), arrhythmias, hypokalemia
- Humans: Visual disturbances (blurring, spots, wavy lines), rashes, GI effects (nausea, vomiting, diarrhea), hepatotoxicity, hypertension/hypotension, tachycardia, peripheral edema, hypokalemia, hypomagnesemia
- Rare (Humans): Eye hemorrhage, anemia, leukopenia, thrombocytopenia, pancytopenia, QT prolongation, nephrotoxicity
Precautions
Feline Warning: Cats are highly susceptible to severe neurologic and systemic adverse effects. Use only as a last resort.
- Hepatic Effects: Can cause liver enlargement and enzyme induction (in dogs). Use with caution in patients with pre-existing hepatic dysfunction.
- Renal Impairment: The IV formulation contains sulfobutyl ether beta-cyclodextrin sodium (SBECD), which can accumulate in patients with reduced renal function.
- Pregnancy: Teratogenic in rats and embryotoxic in rabbits (FDA Category D). Weigh risks vs. benefits in pregnant animals.
- Cardiac: Use with caution in patients with proarrhythmic conditions.
Drug interactions
May increase serum concentrations of these drugs and increase risk for hypoglycemia
Decreased voriconazole concentrations; concurrent use is contraindicated
May increase benzodiazepine concentrations
May increase serum concentrations; dosage adjustment may be required
Decreased voriconazole concentrations; concurrent use is contraindicated
Potential for serious cardiac arrhythmias; concurrent use is contraindicated
Potentially increased AUC for prednisolone
Increased concentrations; decrease cyclosporine dosage by 50% and tacrolimus dosage by 33% when starting voriconazole
May increase plasma concentrations of R-methadone; monitor for toxicity
Can decrease voriconazole concentrations and voriconazole can increase phenytoin concentrations
Potential for serious cardiac arrhythmias; concurrent use is contraindicated
May increase omeprazole (and potentially other PPIs) concentrations
Potential for serious cardiac arrhythmias; concurrent use is contraindicated
Decreased voriconazole concentrations; concurrent use is contraindicated
Possible increased Vinca alkaloid concentrations; monitor for toxicity
May potentiate warfarin's anticoagulant effects
Monitoring
- Clinical efficacy (resolution of fungal infection)
- Liver function tests (baseline and periodic)
- Serum electrolytes (potassium, magnesium)
- Neurologic and visual signs (especially in cats)
Pharmacokinetics
Half-life
Absorption
Dogs: Rapidly and essentially completely absorbed PO; peak levels at ~3 hours. Horses: Well absorbed PO; peak levels at 1-3 hours. Alpacas: Low bioavailability (~24%) after single PO doses.
Distribution
Dogs: Moderately bound to plasma proteins (51%), Vd ~1.3 L/kg. Horses: Low protein binding (31%), Vd 1.35-1.6 L/kg. Distributes well into CSF, tears, and synovial fluid.
Metabolism
Metabolized in the liver to various metabolites; the primary circulating metabolite is the N-oxide metabolite (weak antifungal activity). Dogs exhibit auto-induced metabolism after multiple doses.
Elimination
Dogs: Very complex, dose-dependent non-linear elimination and auto-induction. Horses: Elimination half-life is ~13 hours. Alpacas: Half-life is ~8 hours. Parrots: Short half-life (~1 hour).
Overdose
The minimum lethal dose in rats and mice is 300 mg/kg (4-7X maintenance dose).
- Clinical Signs: Increased salivation, mydriasis, ataxia, depression, dyspnea, and seizures. In human pediatric patients, accidental 5X overdoses caused brief photophobia.
- Treatment: No specific antidote is known. For very large oral overdoses, consider gut emptying (emesis/gastric lavage) followed by close observation and supportive treatment.
Available products
Formulations
- Tablets
- Powder for oral suspension
- Powder for injection (lyophilized)
Veterinary
- None
Human-labeled
- Voriconazole Tablets: 50 mg & 200 mg (Vfend®)
- Voriconazole Powder for Oral Suspension: 45 grams (40 mg/mL after reconstitution) (Vfend®)
- Voriconazole Powder for Injection, Lyophilized: 200 mg in single-use vials (Vfend I.V.®)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Tablets: Store at 15-30°C. Unreconstituted powder for oral suspension: Store in refrigerator (2-8°C). Reconstituted suspension: Store at room temperature (15-30°C); do not refrigerate or freeze; stable for 14 days. Powder for injection: Store at room temperature (15-30°C). Reconstituted injection: Use immediately, or stable for up to 24 hours if refrigerated (2-8°C).
Client information
Voriconazole is an advanced antifungal medication used for severe or resistant fungal infections.
- Investigational Use: This drug is relatively new in veterinary medicine, and its use is often considered investigational. It is also quite expensive.
- Administration: Give the medication at least one hour before or one hour after feeding to ensure proper absorption.
- Adverse Effects to Watch For: Because experience with this drug in pets is limited, monitor your pet closely. Contact your veterinarian immediately if you notice:
- Itching or skin rashes
- Yellowing of the whites of the eyes or gums (jaundice)
- Reduced appetite or weight loss
- Difficulty walking, stumbling, or weakness (especially in the hind legs)
- Vision problems or dilated pupils
- Cats: Cats are particularly sensitive to this medication. Watch them extremely closely for any changes in behavior, appetite, or mobility.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.