Terbinafine

Terbinafine is an antifungal medication of the allylamine class used to treat fungal infections of the skin, nails, and scalp. It is available in oral tablet form and in various topical preparations such as creams, gels, and solutions.

Mechanism of action: Terbinafine inhibits the fungal enzyme squalene epoxidase, blocking ergosterol synthesis. This leads to accumulation of squalene and disruption of the fungal cell membrane, resulting in inhibited growth and death of many dermatophytes.

Spectrum and indications: Terbinafine is highly active against dermatophytes, including Trichophyton species (such as T. rubrum and T. mentagrophytes) and Microsporum species, with variable activity against non-dermatophyte fungi. Topical forms are used for tinea corporis, tinea cruris, and tinea pedis. Oral terbinafine is indicated for onychomycosis (nail infections) and, in some cases, tinea capitis and widespread dermatophytoses; dermal indications may be treated with topical therapy alone.

Administration and pharmacokinetics: Oral terbinafine is typically given as 250 mg once daily. Fingernail infections often require about 6 weeks of treatment, while toenail infections commonly require about 12 weeks. Terbinafine is well absorbed orally and concentrates in keratinizing tissues (skin, nails). It is extensively metabolized in the liver with inactive metabolites and is excreted mainly in the urine and feces. The apparent half-life is long due to tissue accumulation.

Safety and interactions: Common adverse effects include gastrointestinal upset, headache, and rash; dysgeusia (taste disturbance) can occur. Serious but rare hepatic toxicity warrants monitoring during prolonged systemic therapy. It inhibits the CYP2D6 enzyme, which can affect drugs metabolized by this pathway. Caution is advised in liver disease and during pregnancy or breastfeeding; topical formulations generally carry fewer systemic risks.