What is the recommended dose and treatment duration of terbinafine (antifungal medication) for a patient with nail fungus, considering factors such as location of infection and potential liver or renal impairment?

Terbinafine for Nail Fungus: Dosing and Duration

For toenail onychomycosis, prescribe terbinafine 250 mg once daily for 12 weeks; for fingernail onychomycosis, prescribe 250 mg once daily for 6 weeks. 1, 2, 3

Standard Dosing Regimen

• Toenail infections: 250 mg once daily for 12 weeks (may extend to 16 weeks for severe cases) 1, 2
• Fingernail infections: 250 mg once daily for 6 weeks 1, 4, 2
• Can be taken with or without food—absorption is unaffected 1, 2

Why These Durations Matter

Terbinafine persists in nail tissue for up to 30 weeks after treatment cessation, providing continued fungicidal activity even after you stop taking it 1, 2. This pharmacokinetic property allows for relatively short treatment courses compared to older antifungals. The 12-week duration for toenails reflects the slower growth rate and greater thickness of toenails compared to fingernails 5.

Mandatory Pre-Treatment Requirements

Before prescribing terbinafine, you must:

• Confirm mycological diagnosis with both microscopy and culture—do not treat empirically 1
• Obtain baseline liver function tests (ALT and AST) 1, 4, 3
• Obtain complete blood count 1, 2

These baseline tests are non-negotiable because terbinafine can cause hepatotoxicity that may progress to liver failure requiring transplantation 3.

Absolute Contraindications

Do not prescribe terbinafine if the patient has:

• Active or chronic liver disease 1, 4, 2, 3
• Lupus erythematosus 4, 2
• History of allergic reaction to oral terbinafine 3

Special Populations Requiring Dose Adjustment

Pediatric dosing (weight-based): 1, 2

• <20 kg: 62.5 mg daily
• 20-40 kg: 125 mg daily

• 40 kg: 250 mg daily (adult dose)

Renal impairment: No specific dose adjustment is provided in the FDA label, but use caution 3

Hepatic impairment: Terbinafine is contraindicated in active liver disease 3

Critical Safety Monitoring During Treatment

Instruct patients to immediately report and discontinue terbinafine if they develop: 1, 3

• Persistent nausea, anorexia, or fatigue
• Right upper abdominal pain
• Jaundice, dark urine, or pale stools
• Progressive skin rash with scaling or blistering

Additional monitoring considerations:

• Repeat liver function tests if the patient has history of hepatitis, heavy alcohol use, or develops symptoms suggestive of hepatotoxicity 4, 2
• Monitor for taste disturbance (can be severe and permanent—discontinue if occurs) 3
• Monitor for smell disturbance (may be permanent—discontinue if occurs) 3
• Monitor for depressive symptoms 3

Post-Treatment Follow-Up

Re-evaluate the patient 3-6 months after treatment initiation with both mycological analysis (microscopy and culture) and clinical examination of nail growth 1, 4, 2. This timing accounts for the slow growth of nails and the persistent antifungal effect of terbinafine in nail tissue.

Management of Treatment Failure

If the first course fails:

• Restart terbinafine 250 mg daily for the full duration (12 weeks for toenails, 6 weeks for fingernails) without waiting 1, 2
• Confirm the infection is dermatophyte-caused (terbinafine is less effective against Candida) 2
• Repeat baseline liver function tests and complete blood count 2

If the second course fails:

• Switch to itraconazole 200 mg daily for 12 weeks continuously, OR 400 mg daily for 1 week per month for 3 cycles 1, 2

Why Terbinafine is First-Line

Terbinafine demonstrates superior efficacy over all alternatives with complete cure rates at 72 weeks of 55% versus 26% for itraconazole 1. It has fungicidal activity with very low minimum inhibitory concentrations (0.004 μg/mL) against dermatophytes 1, 2. Unlike azole antifungals, terbinafine has minimal drug-drug interactions, making it safer for patients on multiple medications 1, 2.

Common Pitfalls to Avoid

• Do not treat without mycological confirmation—clinical appearance alone is insufficient 1
• Do not skip baseline liver function tests—hepatotoxicity can be severe and unpredictable 3
• Do not use shorter durations for toenails—6 weeks is insufficient for toenail infections (cure rates drop from 71% to 40%) 6, 5
• Do not continue treatment if taste or smell disturbance develops—these can become permanent 3