1. Clinical Overview
Terbinafine hydrochloride is a synthetic allylamine antifungal agent. It is fungicidal against dermatophytes, molds, and certain dimorphic fungi, and fungistatic against yeasts like Candida albicans. In the Indian context, it is a first-line oral treatment for extensive or recalcitrant dermatophytoses (tinea infections) and onychomycosis, which are highly prevalent in the country's tropical climate.
| Onset | Duration | Bioavailability |
|---|---|---|
| Clinical improvement in skin infections is typically seen within 1-2 weeks. For onychomycosis, clinical response is observed after several weeks of therapy, but mycological cure takes longer. | The drug persists in the skin and nails for weeks after discontinuation due to high lipophilicity and keratin affinity, contributing to post-treatment efficacy. | Approximately 70-80% following oral administration. Absorption is slightly increased (by up to 20%) with a high-fat meal. |
2. Mechanism of Action
Terbinafine is a non-competitive, specific inhibitor of the fungal enzyme squalene epoxidase. This enzyme catalyzes the conversion of squalene to squalene 2,3-epoxide in the ergosterol biosynthesis pathway. Inhibition leads to intracellular accumulation of squalene and depletion of ergosterol, an essential component of the fungal cell membrane.
3. Indications & Uses
- Onychomycosis (Fungal infection of fingernails and toenails) due to dermatophytes
- Tinea corporis (Ringworm of the body)
- Tinea cruris (Jock itch)
- Tinea pedis (Athlete's foot)
4. Dosage & Administration
Adult Dosage: For skin infections: 250 mg once daily for 2-4 weeks (T. corporis/cruris: 2 weeks; T. pedis: 2-4 weeks). For fingernail onychomycosis: 250 mg once daily for 6 weeks. For toenail onychomycosis: 250 mg once daily for 12 weeks.
Administration: Tablet should be swallowed whole with a full glass of water. Can be taken with or without food, but taking with food may improve tolerance and slightly enhance absorption. For optimal absorption, consistency in timing relative to meals is advised.
5. Side Effects
Common side effects may include:
- Headache
- Gastrointestinal disturbances (dyspepsia, abdominal pain, nausea, diarrhea, flatulence)
- Taste disturbance (dysgeusia) or loss of taste (ageusia) - usually reversible
- Skin rash (non-serious)
- Pruritus
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Rifampicin | Induces CYP enzymes, increasing terbinafine clearance and reducing plasma levels by ~100%. Risk of therapeutic failure. | Major |
| Cimetidine | Inhibits CYP2C9/CYP3A4, decreasing terbinafine clearance and increasing plasma levels by ~33%. Risk of toxicity. | Moderate |
| SSRIs (e.g., Sertraline, Paroxetine) | CYP2D6 inhibitors. Terbinafine is also a potent CYP2D6 inhibitor, leading to potential increased levels of these drugs and risk of serotonin syndrome. | Major |
| Beta-blockers (e.g., Metoprolol, Propranolol) | Terbinafine (CYP2D6 inhibitor) can significantly increase plasma levels of these drugs, potentiating bradycardia and hypotension. | Major |
| TCAs (e.g., Amitriptyline, Nortriptyline) | Terbinafine inhibits their metabolism via CYP2D6, leading to increased TCA levels and risk of anticholinergic and cardiac side effects. | Major |
| Warfarin | Potential interaction (CYP2C9 inhibition by terbinafine). May increase INR. Close monitoring required. | Moderate |
| Cyclosporine | Terbinafine may reduce cyclosporine metabolism, increasing its levels. Monitor cyclosporine levels and renal function. | Moderate |
7. Patient Counselling
- DO complete the full prescribed course, even if symptoms improve earlier.
- DO take the tablet at the same time each day, with or without food.
- DO inform your doctor about all other medicines, including OTC and herbal products.
- DONT consume alcohol during treatment.
- DONT share your medication with others, even if they have similar symptoms.
- DONT stop the medication without consulting your doctor.
8. Toxicology & Storage
Overdose: Symptoms may include nausea, vomiting, abdominal pain, dizziness, rash, frequent urination, and headache. In massive overdose, CNS depression and hepatotoxicity are potential risks.
Storage: Store below 30°C, in a cool, dry place, protected from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack.