Terbinafine

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.

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)

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

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

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

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

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

Terbinafine is a synthetic allylamine antifungal agent that has established itself as the gold standard for oral treatment of dermatophyte infections, particularly onychomycosis. First developed by Novartis (then Sandoz) in the 1980s and approved by the FDA in 1996, terbinafine introduced a fundamentally different mechanism of action compared to the existing azole antifungals. Its selective inhibition of squalene epoxidase provides potent fungicidal activity against dermatophytes with minimal interference with the mammalian sterol synthesis pathway, resulting in a favorable safety-to-efficacy ratio. The L.I.ON. study and subsequent long-term follow-up data demonstrated mycological cure rates of 76-81% for dermatophyte onychomycosis, with sustained superiority over itraconazole at 5-year follow-up (relapse rate 23% vs. 53%). Available in both oral (250 mg tablets) and topical (1% cream, gel, spray, solution) formulations, terbinafine remains the most prescribed antifungal worldwide for dermatophyte infections. However, the emergence of terbinafine-resistant Trichophyton indotineae has introduced new challenges to its clinical utility.

• Molecular formula: C₂₁H₂₅N·HCl (hydrochloride salt)
• Molecular weight: 327.9 Da (hydrochloride)
• Chemical name: (E)-N-(6,6-dimethyl-2-hepten-4-ynyl)-N-methyl-1-naphthalenemethanamine hydrochloride
• Structural classification: Allylamine antifungal
• Physicochemical properties: White to off-white crystalline powder, freely soluble in methanol and ethanol, slightly soluble in water; highly lipophilic with extensive tissue distribution
• Stability: Store at 25°C (77°F); excursions permitted to 15-30°C

Primary mechanism

• Molecular target: Fungal squalene epoxidase (SE), a non-cytochrome P-450 enzyme catalyzing the conversion of squalene to 2,3-oxidosqualene (squalene epoxide) in the ergosterol biosynthesis pathway
• Binding characteristics: Non-competitive inhibition with Ki = 30 nM for Candida squalene epoxidase; highly selective — inhibition of mammalian (rat liver) SE requires ~2,500-fold higher concentrations (Ki = 77 μM, competitive inhibition)
• Cellular effects: Dual mechanism — (1) ergosterol depletion compromises membrane integrity and increases permeability (fungistatic), and (2) toxic intracellular squalene accumulation disrupts membrane function and cell wall synthesis (fungicidal)
• Selectivity advantage: Unlike azole antifungals, SE is not a cytochrome P-450 enzyme; terbinafine therefore avoids the broad CYP450 inhibition responsible for the extensive drug interaction profile of azoles

Spectrum of activity

• Fungicidal: Dermatophytes (Trichophyton rubrum, T. mentagrophytes, T. tonsurans, Microsporum canis, Epidermophyton floccosum), molds, dimorphic fungi
• Fungistatic: Candida albicans (filamentous form more susceptible than yeast form)
• Resistance: SQLE gene mutations (F397L, L393F, A448T, H440Y, and combinations) confer terbinafine resistance, particularly in Trichophyton indotineae — an emerging global concern requiring antifungal susceptibility testing in refractory cases

FDA-approved indications

• Oral: Onychomycosis of the toenail or fingernail due to dermatophytes (FDA approval 1996)
• Topical (OTC): Tinea pedis (interdigital), tinea corporis, tinea cruris

Evidence-based off-label uses

• Strong evidence:
  • Tinea capitis (first-line for Trichophyton spp.)
  • Extensive or refractory tinea corporis/cruris/pedis
• Moderate evidence:
  • Sporotrichosis (oral, higher doses: 500 mg twice daily)
  • Pityriasis versicolor (topical)
• Limited evidence:
  • Cutaneous candidiasis (topical)
  • Chromoblastomycosis (combination therapy)

Absolute contraindications

• Chronic or active liver disease — idiosyncratic hepatotoxicity risk, including cases of fulminant liver failure
• Known hypersensitivity to terbinafine — risk of anaphylaxis, severe cutaneous adverse reactions

Relative contraindications

• Renal impairment (CrCl ≤50 mL/min) — limited pharmacokinetic data; use with caution
• Pre-existing hepatic impairment (even if not active) — enhanced monitoring required
• Immunocompromised patients — monitor CBC closely; discontinue if neutrophils ≤1000/mm³
• Depression or psychiatric history — monitor for depressive symptoms
• Autoimmune disease — cases of new-onset or exacerbation of cutaneous and systemic lupus erythematosus reported
• Pregnancy — treatment of onychomycosis should be postponed until after delivery

Standard adult dosing

Indication	Dose	Duration
Toenail onychomycosis	250 mg once daily	12 weeks (continuous)
Fingernail onychomycosis	250 mg once daily	6 weeks (continuous)
Tinea corporis/cruris	250 mg once daily	2-4 weeks
Tinea pedis	250 mg once daily	2-6 weeks
Sporotrichosis (off-label)	500 mg twice daily	Until 2-4 weeks after lesion resolution

Pulse dosing (alternative for onychomycosis)

250 mg daily for 4 weeks on, 4 weeks off, then 4 weeks on. Comparable efficacy to continuous dosing in some studies with potentially lower cumulative exposure.

Pediatric dosing (tinea capitis, off-label)

Weight	Daily dose	Duration
<20 kg	62.5 mg	4-6 weeks (Trichophyton); 8-12 weeks (Microsporum)
20-40 kg	125 mg	4-6 weeks (Trichophyton); 8-12 weeks (Microsporum)
>40 kg	250 mg	4-6 weeks (Trichophyton); 8-12 weeks (Microsporum)

Topical formulation (1% cream, gel, spray)

Indication	Application	Duration
Tinea pedis (interdigital)	Once or twice daily	1 week
Tinea corporis/cruris	Once or twice daily	1-2 weeks
Pityriasis versicolor	Once or twice daily	2 weeks

Administration guidelines

• Oral tablets may be taken with or without food (bioavailability ~70%, not significantly affected by meals)
• Topical: clean and dry affected area before application; extend application 1-2 cm beyond the visible border of the lesion

Common adverse reactions (≥1%)

Adverse event	Incidence (oral)	Notes
Headache	12.9%	Usually self-limiting
Diarrhea	5.6%	Dose-related
Rash	5.6%	Discontinue if progressive
Dyspepsia	4.3%	Take with food if problematic
Liver enzyme elevation	3.3%	Monitor LFTs
Taste disturbance	2.8%	May be prolonged or permanent
Pruritus	2.8%
Nausea	2.6%
Abdominal pain	2.4%

Rare but serious adverse reactions (<1%)

• Hepatotoxicity: Fulminant liver failure, including cases requiring transplantation or resulting in death; typically occurs within the first 3 months (median ~30 days); idiosyncratic — can occur without pre-existing liver disease
• Severe cutaneous adverse reactions: Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), DRESS syndrome
• Blood dyscrasias: Severe neutropenia, pancytopenia, agranulocytosis, thrombocytopenia
• Taste/smell disturbance: Ageusia, anosmia — may persist months to years after discontinuation; rare cases are permanent
• Thrombotic microangiopathy (TMA)
• Lupus erythematosus: New onset or exacerbation (cutaneous or systemic)
• Depressive symptoms

Absorption

• Bioavailability: ~70% (oral)
• Time to peak: 1-2 hours
• Food effects: Not clinically significant

Distribution

• Protein binding: >99%
• Tissue distribution: Highly lipophilic; rapidly distributed to dermis, epidermis, sebum, hair, and nails; nail concentrations detectable within 1 week of starting therapy and persist ≥30 weeks after treatment completion
• Volume of distribution: ~2,000 L (extensive tissue binding)

Metabolism

• Primary pathways: Extensive hepatic metabolism via CYP2C9, CYP1A2, CYP3A4, CYP2C8, CYP2C19 to inactive metabolites (at least 15 metabolites identified)
• CYP2D6 inhibition: Terbinafine is a potent inhibitor of CYP2D6 — clinically significant interaction (see Drug Interactions)
• Active metabolites: None with clinically relevant antifungal activity

Elimination

• Half-life: Terminal elimination half-life 200-400 hours (due to slow release from tissues); effective plasma half-life ~36 hours
• Clearance: Plasma clearance ~24 L/h
• Excretion: ~70% renal (as metabolites), remainder fecal

CYP2D6 inhibition (most clinically significant)

Terbinafine is a potent CYP2D6 inhibitor, effectively converting extensive metabolizers to poor metabolizer phenotype:

Drug	Effect	Clinical action
Desipramine	2-fold ↑ Cmax, 5-fold ↑ AUC	Avoid or reduce TCA dose substantially
Other TCAs (nortriptyline, amitriptyline)	Increased levels	Monitor for TCA toxicity
SSRIs (paroxetine, fluoxetine, venlafaxine)	Increased levels	Monitor; consider dose reduction
Beta-blockers (metoprolol, propranolol)	Increased exposure	Monitor heart rate and blood pressure
Class IC antiarrhythmics (flecainide, propafenone)	Risk of toxicity	Use with extreme caution
Codeine, tramadol (prodrugs)	Reduced efficacy	CYP2D6 needed for activation; consider alternative analgesics

Other interactions

• Rifampin: Increases terbinafine clearance by 100% — may reduce efficacy
• Cimetidine: Decreases terbinafine clearance by 33%
• Fluconazole: Increases terbinafine AUC by 69%
• Cyclosporine: Increased cyclosporine clearance — monitor levels
• Warfarin: Monitor INR — reports of altered prothrombin time

Pre-treatment assessment

• Baseline liver function tests (serum transaminases)
• Complete blood count in immunocompromised patients
• Mycological confirmation of diagnosis (KOH, culture, or PCR) before starting oral therapy for onychomycosis
• Medication review for CYP2D6 interactions

Ongoing monitoring

• Repeat LFTs at 4-6 weeks after starting oral therapy
• CBC if treatment exceeds 6 weeks in immunocompromised patients
• Clinical assessment for taste/smell changes, skin reactions, or hepatotoxicity symptoms (fatigue, anorexia, nausea, jaundice, dark urine)
• Discontinue immediately if transaminases exceed 2× upper limit of normal

• Terbinafine achieves the highest mycological cure rates for dermatophyte onychomycosis among available oral antifungals (76-81% in the L.I.ON. study)
• Drug persists in nail tissue ≥30 weeks after completing therapy — mycological cure rates continue to improve after treatment ends
• For tinea capitis caused by Trichophyton spp., terbinafine is first-line; for Microsporum canis, griseofulvin or itraconazole may be preferable due to lower terbinafine efficacy against this species
• The CYP2D6 interaction is the most clinically relevant: desipramine AUC increases 5-fold — always review the medication list before prescribing
• Terbinafine-resistant T. indotineae is an emerging global concern; consider antifungal susceptibility testing in treatment-refractory dermatophytosis and switch to itraconazole or voriconazole if resistance is confirmed
• Terbinafine is not effective for Candida nail infections (fungistatic only against Candida) — azoles are preferred for yeast-related onychomycosis
• Topical terbinafine is available OTC, has shorter treatment courses (1-2 weeks) than azole topicals, and is a safer option during pregnancy