What are the side effects of onychomycosis treatment, particularly with terbinafine?

Side Effects of Onychomycosis Treatment with Terbinafine

Terbinafine commonly causes gastrointestinal side effects (49%) and dermatological reactions (23%), with serious adverse events occurring in only 0.04% of patients. 1

Common Side Effects of Oral Terbinafine

Terbinafine, the first-line treatment for dermatophyte onychomycosis, is generally well tolerated but can cause several side effects:

Gastrointestinal Effects (Most Common - 49%)

• Nausea
• Diarrhea
• Dyspepsia (indigestion)
• Abdominal pain
• Flatulence 1, 2

Dermatological Effects (23%)

• Rash
• Pruritus (itching)
• Urticaria (hives)
• Eczema 1

Other Common Side Effects

• Headache
• Taste disturbance (can be permanent in rare cases)
• Loss of smell (may become permanent) 1, 2

Serious Adverse Effects (Rare)

While serious adverse effects are uncommon (0.04% incidence), patients should be monitored for:

Hepatic Toxicity

• Serious hepatic toxicity occurs rarely, primarily in patients with pre-existing liver disease
• Baseline liver function tests are recommended for:
  • Patients with history of heavy alcohol consumption
  • Patients with hepatitis
  • Patients with haematological abnormalities 1

Severe Cutaneous Reactions

• Stevens-Johnson syndrome
• Toxic epidermal necrolysis 1, 2

Other Serious Reactions

• Lupus-like syndrome
• Photosensitivity reactions
• Depressive symptoms 2

Drug Interactions

Terbinafine has minimal drug-drug interactions compared to azole antifungals. The primary interactions occur with:

• Drugs metabolized by cytochrome P450 2D6 isoenzyme, including:
  • Certain antidepressants
  • Medications for high blood pressure
  • Heart medications
  • Desipramine
  • Caffeine
  • Cyclosporine
  • Fluconazole
  • Rifampin
  • Cimetidine 1, 2

Monitoring Recommendations

Before Starting Treatment

• Baseline liver function tests for high-risk patients
• Complete blood count for patients with history of hematological abnormalities
• Consider baseline monitoring for children (terbinafine is not licensed for pediatric onychomycosis) 1

During Treatment

• Monitor for signs of liver toxicity:
  • Persistent nausea
  • Anorexia
  • Fatigue
  • Vomiting
  • Right upper abdominal pain
  • Jaundice
  • Dark urine
  • Pale stools 2

Side Effects Comparison with Alternative Treatments

Itraconazole

• Similar side effect profile to terbinafine
• Main side effects: headache and gastrointestinal upset
• Additional cautions: heart failure risk, more hepatotoxicity, and more drug interactions 1

Fluconazole

• Similar profile: headache and gastrointestinal upset
• Requires monitoring of liver function tests in high-dose or prolonged therapy 1

Topical Treatments

• Amorolfine: rarely causes local burning, pruritus, and erythema
• Ciclopirox: rarely causes periungual and nail fold erythema
• Tioconazole: can cause allergic contact dermatitis 1

Clinical Pearls and Pitfalls

1. Permanent taste disturbance: Though rare, patients should be warned that taste disturbance can become permanent 1

2. Contraindications: Terbinafine is contraindicated in patients with active or chronic liver disease 1

3. Treatment duration: Side effects may be minimized with shorter treatment courses (6 weeks for fingernails, 12-16 weeks for toenails) 1

4. Patient counseling: Advise patients to report symptoms of liver toxicity, taste/smell disturbance, depressive symptoms, or skin reactions immediately 2

5. Monitoring in special populations: Children require special consideration for baseline monitoring as terbinafine is not licensed for pediatric onychomycosis 1