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terbinafine 250mg tablet clinical pharmacology uses dosage

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Here is a complete clinical profile of Tab Terbinafine 250 mg:

Tab Terbinafine 250 mg

Drug Class: Allylamine antifungal Brand Names: Lamisil, Terbiforce, Terbicip (generic)

Mechanism of Action

Terbinafine inhibits squalene epoxidase, a fungal enzyme essential for ergosterol biosynthesis. This leads to accumulation of squalene (toxic to the fungal cell) and depletion of ergosterol in the fungal cell membrane - resulting in fungicidal activity against dermatophytes. This mechanism differs from azoles, which also target ergosterol but at a later step (lanosterol demethylation).
  • Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 1224
  • Harrison's Principles of Internal Medicine 22E

Pharmacokinetics (ADME)

ParameterDetail
AbsorptionWell absorbed orally; NOT affected by food
Bioavailability~40% (due to first-pass hepatic metabolism)
DistributionHighly lipophilic; accumulates in skin, nails, hair follicles, and fat
Half-lifeInitial ~12-17 h; extends to 200-400 h at steady state (explains persistent nail levels)
MetabolismLiver, primarily via CYP2D6
EliminationMostly in urine
The prolonged accumulation in nails is what makes it effective for onychomycosis even after completing the oral course.

Indications

IndicationDuration
Onychomycosis (toenail) - tinea unguium250 mg once daily x 12 weeks
Onychomycosis (fingernail)250 mg once daily x 6 weeks
Tinea capitis (scalp ringworm)250 mg once daily x 2-4 weeks
Tinea corporis (ringworm)250 mg once daily x 2-4 weeks
Tinea cruris (jock itch)250 mg once daily x 2-4 weeks
Tinea pedis (athlete's foot)250 mg once daily x 2-4 weeks
Terbinafine is considered first-line for dermatophyte infections owing to its fungicidal activity, high efficacy, and favorable pharmacokinetic profile. It is somewhat more effective than itraconazole for onychomycosis.
  • Fitzpatrick's Dermatology, p. 3467
  • Goodman & Gilman's, p. 1224

Drug Interactions

Terbinafine is a potent inhibitor of CYP2D6. This may increase serum levels and toxicity of:
  • Beta-blockers (e.g., metoprolol, propranolol)
  • Tricyclic antidepressants (TCAs)
  • SSRIs
  • MAO inhibitors
  • Class IC and type B antiarrhythmics
Other notable interactions:
  • Rifampin - decreases terbinafine levels (induces clearance)
  • Cimetidine - increases terbinafine levels
  • Cyclosporine - terbinafine increases cyclosporine clearance by ~15%
  • Thioridazine - risk of QT prolongation and arrhythmias
  • Codeine - terbinafine may reduce its efficacy (CYP2D6 converts codeine to morphine)
  • Dermatology 2-Volume Set 5e, p. 2699

Adverse Effects

Common (low incidence):
  • GI distress (nausea, abdominal pain)
  • Headache
  • Rash
  • Taste disturbances (dysgeusia)
Serious (rare):
  • Hepatotoxicity - liver failure (sometimes fatal); LFTs should be checked before starting; contraindicated in active/chronic liver disease
  • Severe neutropenia
  • Stevens-Johnson syndrome (SJS)
  • Toxic epidermal necrolysis (TEN)
  • Depressive symptoms

Contraindications & Special Situations

SituationRecommendation
Active/chronic liver diseaseContraindicated
PregnancyPostpone until after delivery
Renal impairment (marked azotemia)Not recommended; dose adjustment needed
History of allergic reaction to terbinafineContraindicated (risk of anaphylaxis)

Important Limitations

  • Terbinafine penetrates poorly into tissues beyond skin and nails - not preferred for systemic mycoses (e.g., cryptococcosis, aspergillosis)
  • Active primarily against dermatophytes (Trichophyton, Microsporum, Epidermophyton)
  • Less effective against Candida compared to azoles

Monitoring

  • Liver function tests (LFTs) - baseline and if symptoms of hepatotoxicity arise (jaundice, dark urine, fatigue, abdominal pain)
  • CBC - if prolonged therapy (risk of neutropenia)
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