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Triazole antifungal

Fluconazole

Fluconazole is a synthetic triazole antifungal agent with broad-spectrum activity against yeasts and dimorphic fungi. It is widely used for the treatment and prophylaxis of candidiasis and cryptococcal infections. The most testable fact is that fluconazole inhibits fungal cytochrome P450 enzyme 14α-demethylase, disrupting ergosterol synthesis and fungal cell membrane integrity.

Practice questionsHigh-yield facts
Drug classTriazole antifungal
MechanismInhibits fungal CYP51 (14α-demethylase), blocking ergosterol synthesis
Main usesCandidiasis, cryptococcal meningitis, prophylaxis in immunocompromised patients
Key adverse effectHepatotoxicity and QT prolongation
6 min readFor medical studentsUpdated 2026-10-14

Mechanism of action

Fluconazole selectively inhibits fungal cytochrome P450 enzyme 14α-demethylase (lanosterol demethylase, encoded by the CYP51 gene). This enzyme catalyzes the conversion of lanosterol to ergosterol, a critical component of the fungal cell membrane. By blocking this step, fluconazole depletes ergosterol and causes accumulation of toxic sterol precursors, leading to increased membrane permeability, disruption of membrane-bound enzyme function, and inhibition of fungal growth.

The triazole structure provides greater selectivity for fungal CYP enzymes over mammalian cytochrome P450 enzymes compared to older imidazole antifungals, though significant drug interactions still occur. Fluconazole is primarily fungistatic but may be fungicidal at higher concentrations depending on the organism and site of infection.

High-yield for exams
  • Fluconazole inhibits fungal CYP51 (14α-demethylase), blocking ergosterol synthesis in the fungal cell membrane.
  • Excellent CSF penetration makes fluconazole ideal for cryptococcal meningitis consolidation and maintenance therapy.
  • Candida krusei is intrinsically resistant; Candida glabrata often has reduced susceptibility to fluconazole.
  • High-dose fluconazole (≥400 mg/day) in the first trimester is associated with congenital malformations (craniofacial, cardiac, skeletal).
  • Fluconazole inhibits CYP2C9 and CYP2C19, significantly increasing warfarin levels and INR; close monitoring is essential.
  • QT prolongation and hepatotoxicity are serious adverse effects; monitor ECG in at-risk patients and LFTs with prolonged therapy.
  • Oral bioavailability exceeds 90%; absorption is unaffected by food or gastric pH, unlike itraconazole.
  • Dose adjustment required in renal impairment (CrCl <50 mL/min); 80% of fluconazole is excreted unchanged in urine.

Indications

Fluconazole is indicated for a range of fungal infections, particularly those caused by Candida and Cryptococcus species:

Candidiasis

  • Oropharyngeal and esophageal candidiasis, especially in HIV/AIDS patients
  • Vulvovaginal candidiasis (uncomplicated and recurrent)
  • Candidemia and disseminated candidiasis in non-neutropenic patients
  • Candida peritonitis and urinary tract infections

Cryptococcal infections

  • Cryptococcal meningitis: used for consolidation and maintenance therapy after amphotericin B induction in HIV-positive patients
  • Primary prophylaxis against cryptococcosis in patients with advanced HIV

Other indications

  • Prophylaxis of fungal infections in patients undergoing bone marrow transplantation or with prolonged neutropenia
  • Coccidioidomycosis (non-meningeal)
  • Prevention of recurrent candidiasis in immunocompromised hosts

Important limitation: Fluconazole has poor activity against Aspergillus species, Candida krusei (intrinsically resistant), and some strains of Candida glabrata (dose-dependent susceptibility).

Pharmacokinetics

Fluconazole exhibits favorable pharmacokinetic properties that contribute to its widespread clinical use:

Absorption: Oral bioavailability exceeds 90%, with absorption unaffected by food or gastric pH. Peak plasma concentrations occur 1-2 hours after oral administration.

Distribution: The drug has excellent tissue penetration, achieving therapeutic concentrations in cerebrospinal fluid (50-90% of plasma levels), saliva, sputum, urine, skin, and vaginal secretions. This makes it particularly useful for CNS and mucosal infections. Volume of distribution approximates total body water. Plasma protein binding is low (11-12%).

Metabolism: Unlike other azoles, fluconazole undergoes minimal hepatic metabolism. Approximately 80% of the drug is excreted unchanged in urine.

Elimination: The elimination half-life is approximately 30 hours in patients with normal renal function, allowing for once-daily dosing. Renal excretion is the primary route of elimination, necessitating dose adjustment in patients with creatinine clearance below 50 mL/min. In hemodialysis patients, a supplemental dose is required after each dialysis session.

Adverse effects

Fluconazole is generally well tolerated, but several adverse effects warrant attention:

Common adverse effects

  • Gastrointestinal: Nausea, abdominal pain, diarrhea, and vomiting (most frequent)
  • Headache: Occurs in approximately 2-13% of patients
  • Rash: Mild maculopapular rash; more common in HIV-infected individuals

Serious adverse effects

  • Hepatotoxicity: Elevated transaminases, hepatitis, and rarely fulminant hepatic failure. Risk increases with higher doses and prolonged therapy. Monitor liver function tests, especially in patients receiving long-term treatment or those with baseline hepatic impairment.
  • QT prolongation: Fluconazole can prolong the QT interval in a dose-dependent manner, increasing the risk of torsades de pointes, particularly in patients with pre-existing cardiac conditions, electrolyte disturbances (hypokalemia, hypomagnesemia), or concomitant QT-prolonging drugs.
  • Severe cutaneous reactions: Stevens-Johnson syndrome and toxic epidermal necrolysis have been reported, though rare. Patients with serious underlying diseases (AIDS, malignancy) appear to be at higher risk.
  • Teratogenicity: High-dose fluconazole (400-800 mg/day) during the first trimester is associated with a pattern of birth defects including craniofacial, cardiac, and skeletal abnormalities (similar to Antley-Bixler syndrome). Single low-dose therapy (150 mg) for vaginal candidiasis appears to have minimal risk but is generally avoided in pregnancy when possible.

Other adverse effects

  • Hypokalemia
  • Thrombocytopenia and neutropenia (rare)
  • Seizures (rare, more common in patients with predisposing factors)
  • Alopecia with prolonged use

Contraindications and warnings

Absolute contraindications:

  • Hypersensitivity to fluconazole, other azole antifungals, or any component of the formulation
  • Coadministration with terfenadine (in patients receiving fluconazole doses ≥400 mg/day) due to risk of fatal cardiac arrhythmias

Relative contraindications and warnings:

  • Pregnancy: Category C (animal studies show adverse effects). High-dose exposure in the first trimester is associated with congenital malformations. Use only if benefit clearly outweighs risk. Single-dose therapy for vulvovaginal candidiasis should be avoided if possible, though the absolute risk is low.
  • Hepatic impairment: Use with caution and monitor liver function tests. Patients with baseline liver disease are at increased risk of hepatotoxicity.
  • Renal impairment: Dose adjustment is required when creatinine clearance is below 50 mL/min. Fluconazole is removed by hemodialysis.
  • Cardiac conditions: Use cautiously in patients with conditions predisposing to arrhythmias, including congenital or acquired QT prolongation, cardiomyopathy, hypokalemia, or concurrent use of other QT-prolonging medications.
  • Adrenal insufficiency: Rare cases of adrenal insufficiency have been reported with azole antifungals, though less common with fluconazole than with ketoconazole.

Drug interactions

Fluconazole is a potent inhibitor of cytochrome P450 enzymes, particularly CYP2C9, CYP2C19, and to a lesser extent CYP3A4. This results in numerous clinically significant drug interactions:

Major interactions requiring dose adjustment or avoidance

Warfarin: Fluconazole inhibits CYP2C9, significantly increasing warfarin concentrations and INR. Close INR monitoring is essential, and warfarin dose reduction is often necessary.

Phenytoin: CYP2C9 inhibition increases phenytoin levels, potentially causing toxicity. Monitor phenytoin concentrations and adjust dosing accordingly.

Sulfonylureas (glipizide, glyburide): Enhanced hypoglycemic effect due to CYP2C9 inhibition. Monitor blood glucose closely.

Tacrolimus and cyclosporine: Fluconazole increases levels of these calcineurin inhibitors through CYP3A4 inhibition, raising the risk of nephrotoxicity. Dose adjustment and therapeutic drug monitoring are required.

Statins (particularly simvastatin and atorvastatin): Increased statin exposure increases risk of rhabdomyolysis. Consider using pravastatin or rosuvastatin (not metabolized by CYP3A4) or temporarily discontinue statin therapy.

Rifampin: This potent CYP inducer reduces fluconazole concentrations by approximately 25%. Consider increasing fluconazole dose.

Benzodiazepines (midazolam, triazolam): Prolonged sedation due to CYP3A4 inhibition. Use alternative anxiolytics or reduce benzodiazepine dose.

Terfenadine, cisapride, astemizole: Risk of QT prolongation and torsades de pointes. Coadministration is contraindicated (these drugs are largely unavailable in many countries).

Oral contraceptives: Fluconazole may increase ethinyl estradiol and levonorgestrel concentrations, though clinical significance is unclear. Contraceptive efficacy is not compromised.

Voriconazole: Fluconazole increases voriconazole concentrations. Avoid concurrent use.

Clinical pearls

  • CSF penetration: Fluconazole achieves excellent CNS penetration (50-90% of serum concentrations), making it the preferred oral azole for cryptococcal meningitis maintenance therapy and consolidation after amphotericin B induction.
  • Once-daily dosing: The long half-life (30 hours) permits convenient once-daily administration, improving adherence in outpatient settings.
  • Resistance patterns: Candida krusei is intrinsically resistant to fluconazole. Candida glabrata often exhibits dose-dependent susceptibility or resistance, especially in patients with prior fluconazole exposure. Consider susceptibility testing in refractory cases.
  • Loading dose: For invasive candidiasis or cryptococcal meningitis, a loading dose (typically twice the maintenance dose) is used on day 1 to rapidly achieve therapeutic concentrations.
  • Prophylaxis in HIV: Fluconazole is used for primary prophylaxis of cryptococcosis in patients with CD4 counts below 100 cells/μL in some settings, though its routine use is debated due to cost, drug interactions, and resistance concerns. It is also used for secondary prophylaxis (chronic suppressive therapy) after treatment of cryptococcal meningitis until immune reconstitution occurs.
  • Vaginal candidiasis: A single 150 mg oral dose is effective for uncomplicated vulvovaginal candidiasis and is more convenient than topical therapies, though it should be avoided in pregnancy.
  • Therapeutic drug monitoring: Generally not required due to predictable pharmacokinetics and wide therapeutic index, unlike itraconazole and voriconazole.
  • Aspergillus coverage: Fluconazole lacks clinically reliable activity against Aspergillus species. Patients with suspected or confirmed invasive aspergillosis require alternative agents (voriconazole, isavuconazole, or amphotericin B).
  • Bone marrow transplant prophylaxis: Fluconazole reduces the incidence of invasive candidiasis in high-risk patients undergoing allogeneic hematopoietic stem cell transplantation, though breakthrough infections with fluconazole-resistant Candida species can occur.
Mnemonic

FLUCON for fluconazole toxicity: Fulminant hepatitis, Long QT, Uterine (teratogenic), CYP interactions (warfarin, phenytoin), Oral bioavailability excellent, No Aspergillus coverage.

Practice questions

1. A 32-year-old man with AIDS (CD4 count 45 cells/μL) presents with severe headache, fever, and altered mental status. Lumbar puncture reveals increased opening pressure, lymphocytic pleocytosis, elevated protein, and low glucose. India ink stain shows encapsulated yeast. He is started on amphotericin B and flucytosine with clinical improvement after 2 weeks. What is the most appropriate next step in management?

Answer: B. This patient has cryptococcal meningitis. After successful induction therapy with amphotericin B (with or without flucytosine) for at least 2 weeks, the standard of care is to switch to oral fluconazole for consolidation therapy (8 weeks) followed by maintenance therapy until immune reconstitution. Fluconazole's excellent CSF penetration makes it ideal for this indication. Continuing amphotericin B unnecessarily exposes the patient to nephrotoxicity. Discontinuing therapy would lead to relapse.

2. A 45-year-old woman with atrial fibrillation on warfarin develops recurrent vulvovaginal candidiasis and is prescribed fluconazole 150 mg as a single oral dose. Three days later, she presents to the emergency department with gum bleeding and ecchymoses. Her INR is 8.2 (baseline 2.5). Which mechanism best explains this interaction?

Answer: B. Fluconazole is a potent inhibitor of CYP2C9, the enzyme responsible for metabolizing the more active S-enantiomer of warfarin. Inhibition of this enzyme decreases warfarin metabolism, leading to elevated warfarin concentrations and a significantly increased INR. Even a single dose of fluconazole can cause this interaction. CYP2C9 induction would decrease INR, not increase it. While protein binding displacement can occur with some drugs, enzyme inhibition is the primary mechanism here.

3. A 28-year-old woman at 10 weeks gestation presents with severe vulvovaginal pruritus and thick white discharge. Wet mount shows pseudohyphae. She asks about oral treatment because she has difficulty using topical medications. What is the most appropriate management?

Answer: C. Although vulvovaginal candidiasis is common in pregnancy, oral azoles including fluconazole should be avoided, especially in the first trimester. High-dose fluconazole is associated with congenital malformations (craniofacial, cardiac, skeletal defects). Even single low-dose therapy carries some risk and is not recommended. Topical azoles (clotrimazole, miconazole) are safe and effective in pregnancy and are the preferred treatment. Deferring treatment is inappropriate as the infection causes significant discomfort.

Frequently asked questions

How does fluconazole work?

Fluconazole inhibits fungal cytochrome P450 enzyme 14α-demethylase (CYP51), which converts lanosterol to ergosterol, an essential component of the fungal cell membrane. By blocking ergosterol synthesis, fluconazole disrupts membrane integrity and inhibits fungal growth. It is selective for fungal CYP enzymes but still interacts with some human cytochrome P450 enzymes, leading to drug interactions.

What are the side effects of fluconazole?

Common side effects include nausea, headache, abdominal pain, and diarrhea. Serious adverse effects include hepatotoxicity (elevated liver enzymes, hepatitis, rarely fulminant liver failure), QT prolongation with risk of torsades de pointes, and severe cutaneous reactions like Stevens-Johnson syndrome. High-dose use in early pregnancy is associated with birth defects. Patients on long-term therapy require liver function test monitoring.

Is fluconazole safe in pregnancy?

Fluconazole is pregnancy category C and should generally be avoided, especially in the first trimester. High doses (400-800 mg/day) are associated with a pattern of congenital malformations including craniofacial, cardiac, and skeletal defects. Single low-dose therapy (150 mg) for vaginal yeast infections carries lower risk but is still preferably avoided. Alternative topical antifungals are safer options during pregnancy.

Fluconazole vs itraconazole — what is the difference?

Fluconazole has superior oral bioavailability (>90% vs 55% for itraconazole capsules), is unaffected by food or gastric pH, and achieves excellent CSF penetration, making it preferred for cryptococcal meningitis. Itraconazole has broader antifungal activity including Aspergillus species, whereas fluconazole lacks reliable Aspergillus coverage. Itraconazole requires therapeutic drug monitoring due to variable absorption; fluconazole does not. Both inhibit CYP enzymes and cause drug interactions.

Can you drink alcohol while taking fluconazole?

There is no specific interaction between fluconazole and alcohol, and moderate alcohol consumption does not directly interfere with the drug's effectiveness. However, both fluconazole and alcohol can cause liver toxicity independently, so patients with liver disease or those experiencing hepatotoxicity from fluconazole should avoid alcohol. Additionally, if fluconazole is being used to treat an infection, alcohol may impair immune function and delay recovery.

Conditions it is used for

CandidiasisCryptococcal meningitisVulvovaginal candidiasisEsophageal candidiasisCandidemiaOropharyngeal candidiasisCoccidioidomycosisFungal prophylaxis
ItraconazoleVoriconazolePosaconazoleIsavuconazoleAmphotericin BFlucytosineKetoconazoleCaspofungin

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: D01AC15WHO ATC/DDD index: J02AC01

Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.