What is the appropriate first‑line treatment for onychomycosis in a healthy teenage girl without liver disease, pregnancy, or interacting medications?

Treatment for Toenail Fungus in a Teenage Girl

Oral terbinafine is the first-line treatment for toenail onychomycosis in a healthy teenage girl, dosed at 125 mg daily if she weighs 20–40 kg or 250 mg daily if she weighs over 40 kg, given for 12 weeks. 1, 2

Weight-Based Dosing Algorithm

For adolescent patients with confirmed dermatophyte toenail infection:

• < 20 kg: 62.5 mg daily for 12 weeks 1, 2
• 20–40 kg: 125 mg daily for 12 weeks 1, 2
• > 40 kg: 250 mg daily for 12 weeks 1, 2

The British Association of Dermatologists designates both terbinafine and itraconazole as equally strong first-line recommendations in pediatric populations, but terbinafine is generally preferred due to superior efficacy and tolerability. 1, 2

Pre-Treatment Requirements

Before initiating therapy, you must:

• Obtain mycological confirmation through KOH preparation, fungal culture, or nail biopsy to confirm dermatophyte infection and avoid treating non-fungal nail dystrophies 3, 4
• Check baseline liver function tests (ALT, AST) and complete blood count, as terbinafine is unlicensed for pediatric use and requires monitoring 1, 2
• Screen family members for onychomycosis and tinea pedis, as household transmission is common and all infected individuals should be treated simultaneously 1, 2

Why Terbinafine is Preferred in Adolescents

Teenagers achieve higher cure rates than adults because:

• Thinner nail plates allow better drug penetration 2
• Faster nail growth facilitates quicker clearance of infected tissue 2
• Lower drug interaction risk compared to azole antifungals, which is particularly relevant if the patient takes any medications metabolized through CYP450 pathways 5

Adult data supporting terbinafine's superiority show 46% long-term mycological cure versus 13% with itraconazole, with lower relapse rates (23% vs 53%). 2

Alternative First-Line Option: Itraconazole Pulse Therapy

If terbinafine is contraindicated (hepatic impairment) or not tolerated:

• Dose: 5 mg/kg per day for 1 week per month 1, 2
• Duration: Three pulses (3 months total) for toenails 1, 2
• Administration: Take with food in an acidic gastric environment to optimize absorption 1, 2
• Monitoring: Check hepatic function tests in patients with pre-existing liver abnormalities or when continuous therapy exceeds one month 1, 2

Important caveat: Itraconazole carries higher risk of drug-drug interactions via CYP3A4 inhibition and is contraindicated in heart failure. 6

Second-Line Options (When Both First-Line Agents Fail)

Agent	Dose	Duration	Key Limitations
Fluconazole	3–6 mg/kg once weekly	18–26 weeks for toenails	Requires baseline LFTs and CBC; less effective than first-line agents [1,2]
Griseofulvin	10 mg/kg daily (max 500 mg)	Prolonged courses required	Only 30–40% cure rate; take with fatty food [1,2,7]

Fluconazole is reserved for cases where both terbinafine and itraconazole are unsuitable. 2 Griseofulvin should not be used as first-line therapy due to low efficacy and high relapse rates. 1

Common Adverse Effects to Counsel About

With terbinafine, expect:

• Headache and gastrointestinal upset (most common) 1, 2
• Taste disturbance (can rarely be permanent) 1, 6
• Rash, pruritus, urticaria 6
• Potential psoriasis exacerbation or subacute lupus-like syndrome 1

Monitoring During and After Treatment

• Clinical reassessment at 3–6 months after starting therapy 6
• Follow-up for at least 48 weeks from treatment initiation to detect relapse, as recurrence rates for onychomycosis are high (40–70%) 3, 6
• Ongoing hepatic monitoring is only required if pre-existing liver dysfunction exists or symptoms suggest hepatotoxicity 6

Critical Pitfalls to Avoid

• Do not start antifungal therapy without mycological confirmation, as many nail dystrophies are non-fungal and will not respond to treatment 3, 2
• Do not use topical therapy alone in pediatric toenail onychomycosis—there are no pediatric clinical trials demonstrating efficacy, and systemic therapy remains the preferred approach 2
• Do not overlook concomitant tinea pedis or tinea capitis in the affected child, as these require concurrent treatment 2
• Do not forget to decontaminate footwear by applying antifungal powders or terbinafine solution inside shoes to prevent reinfection 1, 3

Prevention Strategies to Reduce Recurrence

• Keep nails trimmed short and straight across 3
• Wear protective footwear in public areas (pools, locker rooms) 1
• Apply absorbent and antifungal powders in shoes regularly 1, 3
• Avoid sharing nail clippers with infected family members 2
• Treat all infected household members simultaneously 1, 2