FAMVIR
Clinical safety rating: caution
Comprehensive clinical and safety monograph for FAMVIR (FAMVIR).
Famciclovir is a prodrug that is rapidly converted to penciclovir, which inhibits viral DNA polymerase by competing with deoxyguanosine triphosphate, thereby inhibiting viral DNA synthesis and replication.
| Metabolism | Rapidly deacetylated and oxidized to the active metabolite penciclovir; primarily metabolized by aldehyde oxidase; penciclovir is phosphorylated by viral thymidine kinase. |
| Excretion | Renal: 60–70% as penciclovir via tubular secretion and glomerular filtration; fecal: <10%; biliary: <1%. |
| Half-life | Terminal elimination half-life of penciclovir is approximately 2–3 hours in patients with normal renal function; extends to 9–18 hours in severe renal impairment (CrCl <30 mL/min). |
| Protein binding | <20% bound to plasma proteins (primarily albumin). |
| Volume of Distribution | Approximately 1.5 L/kg, indicating extensive distribution into tissues, including skin and mucous membranes. |
| Bioavailability | Oral bioavailability of famciclovir as penciclovir is 77% (range 70–85%). |
| Onset of Action | Oral: clinical effect on viral replication within 1–2 hours of administration. |
| Duration of Action | Duration of antiviral effect correlates with intracellular penciclovir triphosphate half-life (7–20 hours in herpes-infected cells); dosing interval is 8 hours for normal renal function. |
| Molecular Weight | 321.3 |
250 mg orally three times daily for 7 days for herpes zoster; 125 mg orally twice daily for 5 days for recurrent genital herpes; 250 mg orally twice daily for 7 days for first-episode genital herpes; 500 mg orally twice daily for 7 days for herpes zoster in immunocompromised patients; 500 mg orally twice daily for 7 days for recurrent mucocutaneous herpes in HIV patients.
| Dosage form | TABLET |
| Renal impairment | For CrCl >= 60 mL/min: standard dosing; CrCl 40-59: 250 mg q12h (zoster) or 125 mg q12h (genital); CrCl 20-39: 250 mg q24h (zoster) or 125 mg q24h (genital); CrCl < 20: 250 mg q48h (zoster) or 125 mg q48h (genital); hemodialysis: same as CrCl < 20, given after dialysis on dialysis days. |
| Liver impairment | Mild to moderate hepatic impairment (Child-Pugh A or B): no adjustment required. Severe hepatic impairment (Child-Pugh C): No specific studies; use with caution, potential for decreased clearance. |
| Pediatric use | Not approved for pediatric use. Safety and efficacy not established; no standard weight-based dosing available. |
| Geriatric use | Elderly patients may have reduced renal function; initiate dosing based on CrCl as per renal adjustment. Monitor renal function regularly. No specific dose adjustment beyond renal-based modification. |
| 1st trimester | Limited human data; animal studies show fetal harm at high doses. Use only if potential benefit justifies risk. |
| 2nd trimester | Limited human data; animal studies show fetal harm at high doses. Use only if potential benefit justifies risk. |
| 3rd trimester | Limited human data; animal studies show fetal harm at high doses. Use only if potential benefit justifies risk. |
Clinical note
Comprehensive clinical and safety monograph for FAMVIR (FAMVIR).
| Placental transfer | Crosses placenta in animal studies; limited human data suggests transfer. |
| Breastfeeding | Excreted into breast milk in low amounts; unlikely to cause adverse effects in infants. Consider benefit vs risk. |
| Lactation Rating |
■ FDA Black Box Warning
None
| Serious Effects |
Hypersensitivity to famciclovir or any component of the formulationHypersensitivity to penciclovir
| Precautions | Acute renal failure due to precipitation in renal tubules, especially in elderly or dehydrated patients; dose adjustment required in renal impairment, Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS) reported in immunocompromised patients, Potential for seizures, hallucinations, or confusion, particularly with high doses or renal impairment |
| Food/Dietary | High-fat meals may delay absorption but do not significantly reduce total exposure; can be taken with or without food. No known food–drug interactions. |
Loading safety data…
| L2 (Safer) |
| Teratogenic Risk | Pregnancy Category B. No evidence of teratogenicity in animal studies. Insufficient human data; use only if clearly needed. First trimester: limited data, no increased malformation risk. Second/third trimester: no known fetal risks. |
| Fetal Monitoring | Monitor renal function (creatinine clearance) in pregnancy due to altered pharmacokinetics. Assess liver function if prolonged therapy. Observe for fetal growth abnormalities via ultrasound if used long-term. |
| Fertility Effects | No evidence of impaired fertility in animal studies. No human data on reproductive function. Theoretical risk of reversible spermatogenesis suppression with high doses. |
| Clinical Pearls |
| Famciclovir is a prodrug of penciclovir; dose adjustment required in renal impairment (CrCl <60 mL/min); not effective for genital herpes suppression at doses <500 mg twice daily; avoid in immunocompromised patients with HIV and CD4+ <100 cells/μL for herpes zoster due to risk of prolonged shedding; can be taken without regard to meals but high-fat meal delays absorption; monitor for thrombotic thrombocytopenic purpura/hemolytic uremic syndrome in immunocompromised patients. |
| Patient Advice | Take famciclovir exactly as prescribed, even if symptoms improve. · Start treatment at the first sign of herpes outbreak for best results. · Do not share the medication with others; it is not a cure for herpes. · Use condoms and avoid sexual activity during outbreaks to reduce transmission. · Report any signs of easy bruising, bleeding, or unusual fatigue. · If you have kidney disease, inform your doctor before starting treatment. |