What is the recommended oral terbinafine regimen for dermatophyte skin and nail infections, including dosage, treatment duration, monitoring, and contraindications?

Oral Terbinafine for Dermatophyte Infections

For dermatophyte nail infections, prescribe terbinafine 250 mg once daily for 6 weeks for fingernails and 12-16 weeks for toenails, with baseline liver function tests and complete blood count required before starting treatment. 1, 2

Dosing Regimen

Adults

• Fingernail onychomycosis: 250 mg daily for 6 weeks 1, 2
• Toenail onychomycosis: 250 mg daily for 12-16 weeks 1, 2
• Can be taken with or without food 2
• Re-evaluate patients 3-6 months after treatment initiation; provide additional treatment if disease persists 1

Children (1-12 years, weight-based)

• <20 kg: 62.5 mg daily
• 20-40 kg: 125 mg daily
• >40 kg: 250 mg daily
• Duration: 6 weeks for fingernails, 12 weeks for toenails 1

Why Terbinafine is First-Line

Terbinafine is superior to itraconazole and should be your default choice based on Grade A evidence showing significantly higher cure rates and lower relapse rates 1. In a landmark multicenter trial of 508 patients, complete cure at 72 weeks was achieved in 55% with terbinafine versus only 26% with pulsed itraconazole 1. More importantly, at 5-year follow-up, mycological cure without retreatment was maintained in 46% of terbinafine patients versus only 13% with itraconazole, with relapse rates of 23% versus 53% respectively 1.

Mandatory Pre-Treatment Monitoring

Obtain baseline liver function tests (ALT and AST) and complete blood count in ALL adult patients before prescribing 1, 2. This is particularly critical for:

• Patients with history of hepatotoxicity or haematological abnormalities 1
• Heavy alcohol consumption 1
• Pre-existing liver disease 1
• Children (as terbinafine is unlicensed for pediatric onychomycosis) 1

Absolute Contraindications

• Active or chronic liver disease 1, 2
• History of allergic reaction to oral terbinafine (risk of anaphylaxis) 2
• Hepatic impairment 1
• Renal impairment (use with caution) 1

Critical Warnings to Discuss with Patients

Potentially Permanent Side Effects

Warn patients explicitly about taste and smell disturbances that can be permanent 2. While uncommon, these are devastating when they occur:

• Taste disturbance/loss: Can be severe, prolonged, or permanent—discontinue immediately if this develops 2
• Smell disturbance/loss: May be prolonged or permanent—discontinue if occurs 2

Serious Hepatotoxicity

Liver failure requiring transplant or resulting in death has occurred 2. Discontinue immediately if signs of liver injury develop:

• Nausea, vomiting, abdominal pain
• Jaundice, dark urine, pale stools
• Unusual fatigue or weakness 2

Other Serious Reactions

• Severe skin reactions: Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS syndrome—discontinue immediately 2
• Depressive symptoms: Monitor for mood changes, worthlessness, sleep pattern changes, loss of interest 2
• Lupus exacerbation: Can aggravate psoriasis or cause subacute lupus-like syndrome 1, 2
• Severe neutropenia: Discontinue if neutrophil count ≤1,000 cells/mm³ 2

Common Adverse Effects

The most frequent side effects (>2% incidence) include 2:

• Headache
• Gastrointestinal upset (diarrhea, nausea, abdominal pain, dyspepsia, flatulence)
• Rash and pruritus
• Liver enzyme abnormalities

Drug Interactions

Terbinafine has minimal drug interactions compared to azoles 1, which is a significant advantage. The primary concern is:

• CYP2D6 substrates: Terbinafine inhibits CYP2D6, affecting metabolism of desipramine, certain antidepressants, antihypertensives, and heart medications 2
• Also interacts with: caffeine, cyclosporine, fluconazole, rifampin, cimetidine 2

Emerging Resistance Concerns

Be aware of emerging terbinafine resistance, particularly with Trichophyton indotineae 3, 4, 5, 6. This is primarily reported from South Asia but cases are appearing globally including Canada 6:

• Resistance rates up to 47.76% in some T. indotineae populations 3
• Associated with SQLE gene mutations (F397L, L393S/F, A448T) 3, 5, 6
• Consider antifungal susceptibility testing if treatment fails 3, 5
• Terbinafine remains highly effective for non-T. indotineae dermatophytes (T. rubrum, T. mentagrophytes, M. canis) 3

Alternative Treatments if Terbinafine Fails or is Contraindicated

1. Itraconazole (Grade A): 200 mg daily for 12 weeks continuous, OR pulse therapy 400 mg daily for 1 week per month (2 pulses for fingernails, 3 for toenails) 1
2. Fluconazole (Grade B): 150-450 mg weekly for 3 months (fingernails) or ≥6 months (toenails)—less effective but useful if terbinafine/itraconazole not tolerated 1

Special Populations

• Pregnancy: Not recommended; unknown if harmful to fetus 2
• Breastfeeding: Terbinafine passes into breast milk—discuss risks/benefits 2
• Photosensitivity: Advise patients to minimize sun exposure and avoid tanning beds 2

Prevention of Recurrence

Counsel patients on 1:

• Always wear protective footwear in public areas (gyms, pools, hotel rooms)
• Apply antifungal powders (miconazole, clotrimazole, tolnaftate) to shoes and feet
• Wear absorbent cotton socks
• Keep nails short
• Don't share nail clippers
• Discard old contaminated footwear or treat with naphthalene mothballs in sealed plastic bags for 3 days
• Treat all infected family members simultaneously