What is the recommended treatment and dosage for oral terbinafine (antifungal medication) in patients with fungal infections such as onychomycosis (fungal infection of the nail)?

Oral Terbinafine Treatment for Onychomycosis

Terbinafine 250 mg daily is the first-line treatment for onychomycosis, administered for 6 weeks in fingernail infections and 12 weeks in toenail infections. 1, 2

Diagnostic Confirmation

Before initiating treatment:

• Obtain mycological confirmation through KOH preparation, fungal culture, or nail biopsy 3
• Identify the causative organism (dermatophytes, Candida, or non-dermatophyte molds) 1

Treatment Algorithm

For Dermatophyte Onychomycosis (most common):

1. First-line therapy: Oral terbinafine 250 mg daily 1, 2

   • Fingernail infection: 6 weeks
   • Toenail infection: 12 weeks
   • Re-evaluate 3-6 months after treatment initiation 1
   • Additional treatment may be needed if disease persists

2. Alternative therapy (if terbinafine is contraindicated):

   • Itraconazole: Either 200 mg daily for 12 weeks continuously OR
   • Pulse therapy: 400 mg daily for 1 week per month (2 pulses for fingernails, 3 pulses for toenails) 1

3. Third-line option:

   • Fluconazole 450 mg weekly (not licensed for onychomycosis)
   • Duration: 3 months for fingernails, at least 6 months for toenails 1

For Candidal Onychomycosis:

• First-line therapy: Itraconazole 400 mg daily for 1 week per month
  • 2 months for fingernail infection
  • 3-4 months for toenail infection 1

Efficacy and Outcomes

• Terbinafine demonstrates superior efficacy compared to itraconazole for dermatophyte infections:
  • Complete cure rates: 55% for terbinafine vs. 26% for itraconazole at 72 weeks 1
  • Long-term mycological cure without relapse: 46% for terbinafine vs. 13% for itraconazole at 5-year follow-up 1
  • Expected cure rates: 80-90% for fingernail infections and 70-80% for toenail infections 1

Pre-Treatment Assessment

• Baseline liver function tests and complete blood count recommended for:
  • Patients with history of heavy alcohol consumption
  • Patients with history of hepatitis
  • Patients with hematological abnormalities
  • Children (terbinafine not licensed for pediatric onychomycosis) 1, 2

Monitoring and Follow-up

• Re-evaluate patients 3-6 months after treatment initiation 1
• Optimal clinical effect is seen months after mycological cure due to time required for healthy nail outgrowth 2
• Monitor for adverse effects, particularly:
  • Gastrointestinal symptoms (most common: nausea, diarrhea)
  • Taste or smell disturbance (can be severe, prolonged, or permanent)
  • Liver abnormalities
  • Skin reactions 2

Common Pitfalls and Caveats

1. Treatment failure may occur due to:

   • Poor compliance
   • Poor drug absorption
   • Immunosuppression
   • Dermatophyte resistance
   • Subungual dermatophytoma preventing drug penetration 3

2. Drug interactions:

   • Terbinafine inhibits CYP450 2D6 isozyme, affecting metabolism of certain drugs
   • However, terbinafine has fewer drug interactions than azole antifungals 2, 4

3. Serious adverse reactions (rare but important):

   • Hepatotoxicity: Discontinue if liver injury develops
   • Taste/smell disturbance: May be permanent
   • Stevens-Johnson syndrome and toxic epidermal necrolysis
   • Neutropenia 2

4. Special populations:

   • Diabetics: Terbinafine preferred due to lower risk of drug interactions 1
   • Immunosuppressed patients: Terbinafine or fluconazole preferred over itraconazole due to fewer interactions with antiretrovirals 1

5. Adjunctive measures to improve outcomes:

   • Keep nails short
   • Disinfect footwear
   • Treat concomitant tinea pedis
   • Treat infected family members 3

Terbinafine's fungicidal mechanism of action and its persistence in nail tissue for up to 30 weeks after treatment cessation contribute to its superior efficacy compared to other antifungals 4, 5. This persistence allows for complete nail regrowth even after the medication is discontinued.