1. Clinical Overview
Famciclovir is an oral prodrug antiviral agent of the guanine nucleoside analogue class. It is rapidly converted in vivo to its active metabolite, penciclovir, which selectively inhibits viral DNA replication in herpesvirus-infected cells. It is a cornerstone therapy for herpes zoster (shingles) and recurrent episodes of genital herpes in immunocompetent patients, and for the suppression of recurrent genital herpes. It is also used for herpes labialis (cold sores) and for the management of herpesvirus infections in immunocompromised patients.
| Onset | Duration | Bioavailability |
|---|---|---|
| Rapid absorption; penciclovir triphosphate formation within infected cells begins within hours of administration. | The intracellular half-life of penciclovir triphosphate in HSV-1, HSV-2, and VZV-infected cells is prolonged (10-20 hours), allowing for less frequent dosing compared to acyclovir. | Approximately 77% following oral administration. |
2. Mechanism of Action
Famciclovir is a prodrug that is rapidly converted to penciclovir. Penciclovir is phosphorylated by viral thymidine kinase (in infected cells) to penciclovir monophosphate, and subsequently by cellular kinases to penciclovir triphosphate. This active form competitively inhibits viral DNA polymerase, thereby inhibiting viral DNA synthesis and replication.
3. Indications & Uses
- Acute Herpes Zoster (Shingles)
- Recurrent Genital Herpes (Treatment of episodes)
- Suppression of Recurrent Genital Herpes
4. Dosage & Administration
Adult Dosage: Herpes Zoster: 500 mg every 8 hours for 7 days. Recurrent Genital Herpes: 125 mg twice daily for 5 days OR 1000 mg twice daily for 1 day. Suppression of Genital Herpes: 250 mg twice daily. Herpes Labialis: 1500 mg as a single dose.
Administration: May be taken with or without food. Swallow tablet whole with a glass of water. For optimal results in treating an episode, initiate therapy at the earliest sign or symptom (e.g., prodrome of tingling, itching, burning).
5. Side Effects
Common side effects may include:
- Headache
- Nausea
- Diarrhea
- Fatigue
- Pruritus (itching)
- Flatulence
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Probenecid | Increases plasma concentration of penciclovir by reducing its renal tubular secretion. May increase risk of side effects. | Moderate |
| Other nephrotoxic drugs (e.g., Aminoglycosides, NSAIDs, Cyclosporine) | Increased risk of renal dysfunction. Monitor renal function closely. | Moderate |
| Digoxin | Famciclovir may slightly increase digoxin levels. Monitor digoxin levels. | Mild |
| Theophylline | May slightly decrease theophylline clearance. Clinical significance is likely minimal. | Mild |
7. Patient Counselling
- DO start medication at the first sign of tingling, itching, or burning for recurrent episodes.
- DO complete the full course of therapy even if symptoms improve.
- DO inform your doctor about all other medications you are taking.
- DO maintain adequate hydration.
- DONT share your medication with others.
- DONT engage in sexual activity during an active genital herpes outbreak to prevent transmission.
8. Toxicology & Storage
Overdose: Symptoms may include nausea, vomiting, headache, confusion, hallucinations, and renal impairment. Acute renal failure has been reported with very high doses.
Storage: Store at room temperature (15-30°C), protected from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack.