What oral medication is recommended for treating onychomycosis in adults?

Oral Medication for Nail Fungus

Terbinafine 250 mg daily is the first-line oral treatment for nail fungus (onychomycosis), taken for 6 weeks for fingernails or 12-16 weeks for toenails, based on its superior cure rates and lower relapse rates compared to all alternatives. 1

Treatment Algorithm

First-Line: Terbinafine

• Dosing: 250 mg once daily
  • Fingernails: 6 weeks
  • Toenails: 12-16 weeks
• Efficacy: Achieves complete cure in 55% of patients at 72 weeks, with only 23% mycological relapse and 21% clinical relapse at 5-year follow-up 1
• Advantages: Minimal drug interactions (only CYP2D6 substrates), better tolerability, superior long-term outcomes 1

Second-Line: Itraconazole

Use when terbinafine is contraindicated (active liver disease) or not tolerated 1

Two dosing options:

• Continuous: 200 mg daily for 12 weeks
• Pulse therapy: 400 mg daily for 1 week per month
  • 2 pulses for fingernails
  • 3 pulses for toenails

Important limitation: Only 26% complete cure rate with pulse therapy versus 55% for terbinafine, with significantly higher relapse (53% mycological, 48% clinical) 1

Third-Line: Fluconazole (Off-Label)

Reserve for patients unable to tolerate both terbinafine and itraconazole 1

• Dosing: 450 mg once weekly
  • Fingernails: 3 months
  • Toenails: minimum 6 months
• Efficacy: Lower than terbinafine or itraconazole (28-36% clinical cure for toenails)
• Advantage: Once-weekly dosing may improve compliance; fewer drug interactions than itraconazole 1

Critical Monitoring Requirements

Before Starting Treatment

Terbinafine 1:

• Baseline liver function tests (LFTs) and complete blood count (CBC) required for:
  • History of heavy alcohol consumption
  • Hepatitis or liver disease
  • Haematological abnormalities
  • All pediatric patients

Itraconazole 1:

• Monitor LFTs in patients with:
  • Pre-existing abnormal liver function
  • Continuous therapy >1 month
  • Concomitant hepatotoxic drugs

Fluconazole 1:

• Baseline LFTs and CBC
• Adjust dose for renal impairment (predominantly renally excreted)

During Treatment

Re-evaluate patients 3-6 months after treatment initiation; continue treatment if disease persists 1

Common Pitfalls and Contraindications

Terbinafine

• Contraindicated: Active or chronic liver disease 1
• Warn patients: Taste disturbance can be permanent (rare but critical counseling point) 1
• Can aggravate: Psoriasis, may cause subacute lupus-like syndrome 1
• Common side effects: Headache, gastrointestinal upset, taste disturbance (49% GI, 23% dermatological in post-marketing surveillance) 1

Itraconazole

• Contraindicated: Heart failure 1
• Requires: Food and acidic pH for optimal absorption 1
• More drug interactions: Strong CYP450 inhibitor 1
• Common side effects: Headache, gastrointestinal upset 1

Fluconazole

• High discontinuation rate: 58% for 300-450 mg weekly doses due to adverse effects 1
• Common side effects: Headache, rash, GI complaints, insomnia 1

Why Terbinafine Wins

The evidence is unequivocal: A large multicenter randomized trial of 508 patients with 5-year follow-up demonstrated terbinafine's superiority—46% maintained mycological cure without retreatment versus only 13% with itraconazole 1. Terbinafine is superior both in vitro and in vivo for dermatophyte onychomycosis (the most common cause), with lower relapse rates and fewer drug interactions 1.

The British Association of Dermatologists gives both terbinafine and itraconazole "A" level recommendations but explicitly states terbinafine is "generally preferred over itraconazole" based on higher efficacy and tolerability 1.