1. Clinical Overview
Famciclovir is an oral prodrug antiviral agent of penciclovir, a guanosine analogue. It is indicated for the treatment of acute herpes zoster (shingles) and for the treatment or suppression of recurrent episodes of genital herpes in immunocompetent patients. It is also used for the management of herpes labialis (cold sores). Famciclovir is rapidly converted to penciclovir in the body, which inhibits viral DNA synthesis and replication.
| Onset | Duration | Bioavailability |
|---|---|---|
| Plasma concentrations of penciclovir are detectable within 15-30 minutes of oral administration. | The intracellular half-life of penciclovir triphosphate in HSV-1, HSV-2, and VZV infected cells is 10-20 hours, 20 hours, and 7 hours, respectively, allowing for less frequent dosing. | Approximately 77% (oral bioavailability of penciclovir from famciclovir). |
2. Mechanism of Action
Famciclovir is a prodrug of penciclovir. After oral administration, it is rapidly converted to penciclovir. Penciclovir is phosphorylated by viral thymidine kinase to penciclovir monophosphate, which is then further phosphorylated 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 in Immunocompetent Patients
4. Dosage & Administration
Adult Dosage: Herpes Zoster: 500 mg every 8 hours for 7 days. Recurrent Genital Herpes (Treatment): 125 mg twice daily for 5 days. Suppression of Genital Herpes: 250 mg twice daily for up to 1 year. Herpes Labialis: 1500 mg as a single dose.
Administration: Can be taken with or without food. Swallow the tablet whole with a glass of water. For herpes zoster, initiate therapy at the earliest sign or symptom (within 72 hours of rash onset is most effective). For recurrent episodes, initiate at the first sign or symptom.
5. Side Effects
Common side effects may include:
- Headache
- Nausea
- Diarrhea
- Fatigue
- Pruritus (itching)
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 |
| Digoxin | Famciclovir may slightly increase the plasma concentration of digoxin. Monitoring of digoxin levels is advised. | Moderate |
| Theophylline | Potential for increased theophylline levels; clinical significance unknown. Monitor for toxicity. | Low |
| Other drugs eliminated by active renal tubular secretion (e.g., cimetidine) | Potential for increased plasma levels of either drug. | Low |
7. Patient Counselling
- DO start medication at the earliest sign of an outbreak (tingling, itching, pain).
- DO complete the full course of treatment 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 use it to treat infections other than those prescribed.
8. Toxicology & Storage
Overdose: Symptoms may include nausea, vomiting, headache, confusion, seizures (in extreme cases), and renal impairment.
Storage: Store below 30°C. Protect from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack.