Terbinafine

What is it?

Terbinafine is a synthetic antifungal medication from the allylamine class, available in both oral and topical formulations. It works by blocking squalene epoxidase, a key enzyme in the production of ergosterol (an essential component of fungal cell membranes), leading to fungal cell death. Terbinafine is the most effective oral antifungal for dermatophyte infections and represents the first-line treatment for onychomycosis (fungal nail infections). Unlike azole antifungals, terbinafine has a highly selective mechanism that does not interfere with human cholesterol synthesis, resulting in a favorable safety profile for most patients.

What are the indications?

Terbinafine is effective against a wide range of fungal skin and nail infections:

• Main indications:
  • Onychomycosis (fungal nail infection) caused by dermatophytes — the primary approved indication for oral terbinafine
  • Tinea pedis (athlete's foot) — both topical and oral formulations
  • Tinea corporis (ringworm) and tinea cruris (jock itch)
• Secondary indications:
  • Tinea capitis (scalp ringworm) — first-line oral treatment for Trichophyton species in children and adults
  • Pityriasis versicolor (topical formulation)
  • Cutaneous candidiasis (topical formulation)

How does it work?

Terbinafine eliminates fungi through a dual mechanism targeting ergosterol synthesis:

• Primary mechanism: Selective, non-competitive inhibition of fungal squalene epoxidase, blocking an early step in ergosterol biosynthesis; this depletes ergosterol from the fungal cell membrane and causes toxic accumulation of squalene inside the fungal cell, leading to cell death
• Therapeutic effect: Fungicidal activity against dermatophytes (Trichophyton, Microsporum, Epidermophyton), meaning it kills fungi rather than merely inhibiting their growth; the drug accumulates in skin, nails, and hair, providing prolonged antifungal activity even after treatment ends
• Onset of action: Clinical improvement of skin infections within 1-2 weeks; for nail infections, the healthy nail must grow out completely — fingernails require approximately 6 months, toenails 12-18 months for full cosmetic improvement

How is it used?

Treatment protocols vary by formulation and indication:

• Oral terbinafine (250 mg tablets):
  • Toenail onychomycosis: 250 mg once daily for 12 weeks
  • Fingernail onychomycosis: 250 mg once daily for 6 weeks
  • Tinea corporis/cruris: 250 mg once daily for 2-4 weeks
  • Tinea pedis: 250 mg once daily for 2-6 weeks
  • Tinea capitis (children): weight-based dosing for 4-6 weeks (Trichophyton) or 8-12 weeks (Microsporum)
• Topical terbinafine (1% cream, gel, or spray):
  • Tinea pedis/corporis/cruris: apply once or twice daily for 1-2 weeks
  • Pityriasis versicolor: apply once or twice daily for 2 weeks
• What to expect: Skin infections improve within 1-2 weeks; nail infections require months for the healthy nail to grow out completely after the treatment course ends

What are the adverse reactions?

Terbinafine is generally well tolerated, with adverse reactions more common with oral than topical use:

• Common reactions (oral formulation):
  • Gastrointestinal symptoms (diarrhea, nausea, abdominal pain, dyspepsia)
  • Headache
  • Skin rash or pruritus
• Less common reactions (oral formulation):
  • Taste disturbance (dysgeusia or complete loss of taste) — may persist for weeks to months after stopping treatment and in rare cases is permanent; report immediately if it occurs
  • Liver enzyme elevations (transaminases) — usually mild and self-limiting
  • Smell disturbance (hyposmia or anosmia)
• Serious reactions (rare but important):
  • Hepatotoxicity — liver failure has been reported, typically within the first 3 months; requires baseline and follow-up liver function tests
  • Severe skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS syndrome)
  • Blood dyscrasias (severe neutropenia, pancytopenia, agranulocytosis)
• Common reactions (topical formulation):
  • Local irritation, burning, or redness at the application site — generally mild and self-limiting

Who should not use it?

Terbinafine requires careful patient selection, particularly for oral use:

• Absolute contraindications:
  • Chronic or active liver disease
  • Known hypersensitivity to terbinafine
• Relative contraindications:
  • Renal impairment (creatinine clearance ≤50 mL/min) — limited data, use with caution
  • Depression or psychiatric history — monitor for mood changes
  • Immunocompromised patients — monitor complete blood count if treatment exceeds 6 weeks
• Special precautions:
  • Pregnancy — do not initiate oral terbinafine during pregnancy; nail infections can be treated after delivery; topical formulation has minimal systemic absorption and is considered safer
  • Breastfeeding — terbinafine is excreted in breast milk; avoid oral use during breastfeeding
  • Concomitant use of CYP2D6 substrates (tricyclic antidepressants, certain beta-blockers, SSRIs) — terbinafine is a potent CYP2D6 inhibitor and can significantly increase levels of these medications

Tips for getting the most out of treatment

Maximizing therapeutic benefits:

• Complete the full treatment course even if symptoms resolve early — premature discontinuation increases recurrence risk
• For nail infections, confirm the diagnosis with mycological testing (KOH preparation or culture) before starting oral therapy
• Oral terbinafine can be taken with or without food — absorption is not significantly affected by meals

Minimizing adverse reactions:

• Obtain baseline liver function tests before starting oral therapy and repeat at 4-6 weeks
• Discontinue immediately and seek medical advice if taste or smell changes occur
• Review all current medications for CYP2D6 interactions before starting treatment

Lifestyle considerations:

• Maintain good foot hygiene — dry feet thoroughly, especially between toes, and use breathable footwear
• Treat all affected sites simultaneously to prevent reinfection (e.g., feet and nails together)
• Discard old shoes or treat them with antifungal spray to prevent reinfection after completing therapy