What is the treatment regimen for a patient with a fungal infection using terbinafine, considering potential interactions and precautions?

Terbinafine Treatment Guidelines

Standard Dosing and Duration

Terbinafine 250 mg once daily is the first-line treatment for dermatophyte onychomycosis, with 6 weeks for fingernails and 12 weeks for toenails. 1

• Fingernail onychomycosis: 250 mg daily for 6 weeks 1, 2
• Toenail onychomycosis: 250 mg daily for 12 weeks (up to 16 weeks for severe cases) 1, 2
• Tinea capitis (scalp ringworm): Highly effective for Trichophyton species at 2-4 weeks, but significantly less effective for Microsporum species (use griseofulvin instead) 2

The drug can be taken with or without food, as absorption is unaffected by food intake. 1, 2

Why Terbinafine is First-Line Therapy

Terbinafine should be considered first-choice therapy based on its superior efficacy and tolerability compared to all alternatives. 1

• It is the only oral fungicidal antimycotic, inhibiting squalene epoxidase which depletes ergosterol and causes toxic squalene accumulation 1, 3
• Achieves mycological cure rates of 70-80% for toenails and 80-90% for fingernails 1, 3
• Demonstrates significantly higher complete cure rates (55%) compared to itraconazole (26%) at 72 weeks 1
• Has lower relapse rates: 23% for terbinafine versus 53% for itraconazole at 5-year follow-up 1
• Persists in nails for 6 months after treatment completion due to lipophilic properties and long half-life 1, 2, 3

Pre-Treatment Requirements

Mycological confirmation is mandatory before initiating treatment—this is the most common cause of treatment failure when omitted. 1, 3

• Obtain KOH preparation, fungal culture, or nail biopsy to confirm dermatophyte infection 3
• Baseline liver function tests (ALT and AST) are required for all patients 1, 2, 4
• Complete blood count is required before treatment initiation 1, 2
• Additional baseline monitoring is recommended for patients with history of hepatitis, heavy alcohol consumption, or hematological abnormalities 1

Absolute Contraindications

Do not prescribe terbinafine in patients with active or chronic liver disease. 1, 2

• Active or chronic liver disease is an absolute contraindication 1, 2
• Lupus erythematosus is an absolute contraindication 2
• Allergy to oral terbinafine 4

Pediatric Dosing

Terbinafine is not FDA-approved for pediatric onychomycosis, though it is highly effective for tinea capitis caused by Trichophyton species. 2, 5

Weight-based dosing for children: 2

• <20 kg: 62.5 mg daily
• 20-40 kg: 125 mg daily
• >40 kg: 250 mg daily (adult dose)

Duration: 6 weeks for fingernails, 12 weeks for toenails 2

For tinea capitis, treatment duration is 2-4 weeks for Trichophyton infections, but griseofulvin remains superior for Microsporum infections. 1, 2

Drug Interactions and Safety Profile

Terbinafine has minimal drug-drug interactions compared to azole antifungals, making it safer for patients on multiple medications. 1, 2

• The only potentially significant interaction involves drugs metabolized by cytochrome P450 2D6 (certain antidepressants, beta-blockers, antiarrhythmics) 1, 2
• No interaction with corticosteroids—these can be safely co-prescribed 2
• Preferred over itraconazole for diabetic patients due to no hypoglycemia risk 3
• Preferred for immunocompromised patients due to lower risk of interactions with antiretrovirals and immunosuppressants 3

Common Adverse Effects

The incidence of serious adverse events is only 0.04%. 1, 2

Most common side effects: 1

• Gastrointestinal complaints (49%): nausea, diarrhea, abdominal pain
• Dermatological events (23%): rash, pruritus, urticaria, eczema
• Headache
• Taste disturbance (can be permanent—warn patients) 1, 4

Rare but serious adverse reactions: 1, 3, 4

• Stevens-Johnson syndrome and toxic epidermal necrolysis
• Severe hepatotoxicity (usually in patients with pre-existing liver disease)
• Subacute lupus-like syndrome
• Permanent loss of taste or smell

Post-Treatment Evaluation and Management of Treatment Failure

Re-evaluate patients 3-6 months after treatment initiation, as optimal clinical effect occurs months after mycological cure due to time required for healthy nail outgrowth. 1, 2, 4

If disease persists at 3-6 months: 2, 3

• Confirm dermatophyte infection through repeat culture
• Evaluate for dermatophytoma (tightly packed fungal mass), which may require partial nail removal 1, 3
• Restart terbinafine immediately without additional waiting period if dermatophyte is confirmed 2
• Consider itraconazole (200 mg daily for 12 weeks or 400 mg daily for 1 week/month for 3 pulses) only after second terbinafine failure 2

The clinical relapse rate at 6+ months after achieving clinical cure is approximately 15%. 4

Critical Pitfalls to Avoid

• Never treat without mycological confirmation—this is the most common cause of treatment failure 1, 3
• Do not use griseofulvin unless all other options are contraindicated—it has only 30-40% mycological cure rates and requires 12-18 months of treatment 1, 3
• Warn patients about permanent taste disturbance risk before initiating therapy 1, 4
• Do not prescribe for Candida onychomycosis—terbinafine has lower fungistatic activity against Candida species; itraconazole is preferred 1
• Always address concurrent tinea pedis and implement preventive measures to prevent recurrence 3

Special Populations

Pregnancy and breastfeeding: 4

• Should not start terbinafine during pregnancy without discussing with physician
• Terbinafine passes into breast milk and may harm the baby

Renal/hepatic impairment: 1

• Terbinafine clearance is decreased in severe liver or kidney disease
• Requires dose adjustment or alternative therapy