1. Clinical Overview
Terbinafine is a synthetic allylamine antifungal agent, available as a 500mg oral tablet. It is fungicidal and is considered a first-line systemic treatment for dermatophyte infections (onychomycosis, tinea capitis, tinea corporis) in India due to its high efficacy, favorable pharmacokinetics, and cost-effectiveness in generic forms. It works by inhibiting squalene epoxidase, a key enzyme in fungal ergosterol synthesis.
| Onset | Duration | Bioavailability |
|---|---|---|
| Clinical improvement typically seen within 1-2 weeks for cutaneous infections; nail growth improvement takes 6-12 weeks. | Prolonged post-treatment effect due to high affinity for keratinous tissues (skin, nails, hair) and lipophilic nature, persisting for weeks after cessation. | Approximately 70-80% in fasting state. Absorption increases by ~20% with a high-fat meal. |
2. Mechanism of Action
Terbinafine is a fungicidal agent that selectively inhibits the fungal enzyme squalene epoxidase. This inhibition occurs early in the ergosterol biosynthesis pathway. Ergosterol is a critical component of the fungal cell membrane, providing structural integrity and function.
3. Indications & Uses
- Onychomycosis (Fungal infection of fingernails and toenails) caused by dermatophytes
- Tinea Capitis (Scalp ringworm)
- Extensive Tinea Corporis (Body ringworm) or Tinea Cruris (Jock itch) not responsive to topical therapy
4. Dosage & Administration
Adult Dosage: Onychomycosis: 500mg once daily. Fingernails: 6 weeks. Toenails: 12 weeks. Tinea Capitis/Corporis: 250mg once daily for 2-4 weeks; severe cases may require 500mg daily.
Administration: Take tablet with a full glass of water. Can be taken with or without food, but taking with food may improve tolerance and slightly increase absorption. Swallow whole; do not crush or chew.
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, pruritus
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Rifampicin | Induces CYP enzymes, increasing terbinafine clearance and reducing plasma levels by ~50%. May lead to therapeutic failure. | Major |
| Cimetidine | Inhibits CYP enzymes, decreases terbinafine clearance, increasing plasma levels by ~33%. Risk of increased side effects. | Moderate |
| SSRIs (e.g., Sertraline, Paroxetine) | Terbinafine is a potent inhibitor of CYP2D6. Increases plasma levels of CYP2D6 substrates (many SSRIs, TCAs, beta-blockers like metoprolol, codeine). Can lead to toxicity of the co-administered drug. | Major |
| Warfarin | Potential interaction via CYP inhibition. Monitor INR closely as terbinafine may potentiate warfarin effect. | Moderate |
| Cyclosporine | Terbinafine may reduce cyclosporine metabolism, increasing its levels. Monitor cyclosporine levels and renal function. | Moderate |
| Oral Contraceptives | No clinically significant interaction reported. Efficacy not affected. | Minor |
7. Patient Counselling
- DO complete the full prescribed course even if symptoms improve earlier.
- DO take with food if you experience stomach upset.
- DO report any signs of liver problems: unusual fatigue, nausea, vomiting, right upper abdominal pain, jaundice (yellowing of skin/eyes), dark urine.
- DO inform all your doctors and pharmacists you are taking terbinafine.
- DONT consume alcohol during and for 2 weeks after treatment.
- DONT take other over-the-counter medicines (especially paracetamol in high doses) without consulting your doctor.
- DONT stop the medication without consulting your doctor.
8. Toxicology & Storage
Overdose: Nausea, vomiting, abdominal pain, dizziness, rash, frequent urination, headache. In massive overdose, CNS depression and hepatorenal toxicity are possible.
Storage: Store below 30°C, in a cool, dry place. Protect from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack.