Terbinafine Dosing for Pediatric Tinea Unguium
For pediatric patients with tinea unguium (onychomycosis), terbinafine is dosed weight-based: 62.5 mg daily for children <20 kg, 125 mg daily for 20-40 kg, and 250 mg daily for >40 kg, given for 6 weeks for fingernail infections and 12 weeks for toenail infections. 1
Weight-Based Dosing Algorithm
The British Association of Dermatologists guidelines provide clear, evidence-based dosing for children ages 1-12 years:
- <20 kg body weight: 62.5 mg once daily
- 20-40 kg body weight: 125 mg once daily
- >40 kg body weight: 250 mg once daily
Duration by infection site:
- Fingernail onychomycosis: 6 weeks
- Toenail onychomycosis: 12 weeks 1
Important Clinical Considerations
Terbinafine is the preferred first-line agent over itraconazole for pediatric dermatophyte onychomycosis, despite being unlicensed for use in children. 1 The FDA label confirms that safety and efficacy have not been formally established in pediatric patients with onychomycosis 2, but extensive clinical experience supports its use.
Mandatory Monitoring
Obtain baseline laboratory tests before initiating therapy:
- Liver function tests (AST, ALT)
- Complete blood count with differential 1
This monitoring is particularly critical in pediatrics given the off-label nature of use. The guidelines emphasize this requirement specifically for pediatric patients.
Key Safety Warnings
Hepatotoxicity: Liver failure requiring transplantation or resulting in death has occurred with oral terbinafine. Discontinue immediately if clinical signs of liver disease develop (persistent nausea, anorexia, fatigue, jaundice, right upper quadrant pain, dark urine). 2
Taste and smell disturbances: Can be severe, prolonged, or permanent. Discontinue if these occur. 2
Neutropenia: Severe cases reported. Stop therapy if neutrophil count drops to ≤1,000 cells/mm³. 2
Alternative Agents (Second-Line)
If terbinafine is contraindicated or not tolerated:
Itraconazole (pulse therapy):
- 5 mg/kg/day for 1 week per month
- 2 pulses for fingernails, 3 pulses for toenails
- Take with food for optimal absorption 1
Fluconazole:
- 3-6 mg/kg once weekly
- 12-16 weeks for fingernails, 18-26 weeks for toenails 1
Common Pitfalls to Avoid
Treating without mycological confirmation: Other conditions (psoriasis, traumatic nail dystrophy) can mimic onychomycosis. Always confirm diagnosis with KOH preparation or fungal culture before initiating systemic therapy. 3
Inadequate treatment duration: The full 12-week course for toenails is essential. Shorter durations significantly reduce cure rates—studies show 40% cure with 6 weeks versus 71% with 12 weeks. 4
Missing family screening: Onychomycosis and tinea pedis are contagious. Examine and treat all infected family members simultaneously to prevent reinfection. 1
Ignoring drug interactions: Terbinafine is a potent CYP2D6 inhibitor. Review all concurrent medications, particularly those metabolized by CYP2D6. 2
Using higher doses without evidence: A 500 mg dose in adults showed no additional benefit over 250 mg 5, and there is no pediatric data supporting doses above the weight-based recommendations.
Food Administration
Terbinafine can be taken with or without food—food increases absorption by less than 20%, which is not clinically significant. 2