famciclovir
Brand: Famvir
Prototype: acyclovir
Drug Class: antiviral — penciclovir prodrug
Drug Family: antiviral
Subclass: oral prodrug of penciclovir
Organ Systems: infectious-disease
Mechanism of Action
Prodrug converted to penciclovir, which is phosphorylated by viral TK to penciclovir triphosphate; inhibits viral DNA polymerase with longer intracellular half-life than acyclovir triphosphate, potentially allowing less frequent dosing.
herpesvirus thymidine kinaseviral DNA polymerase
Indications
- herpes zoster (shingles)
- genital herpes — initial and recurrent episodes and suppression
- herpes labialis (cold sores) in immunocompetent adults
Contraindications
- famciclovir/penciclovir hypersensitivity
Adverse Effects
Common
- headache
- nausea
- diarrhea
- fatigue
Serious
- nephrotoxicity (less common than acyclovir)
- CNS effects (headache, confusion at high doses)
Pharmacokinetics (ADME)
| Absorption | 77% bioavailability; converted to penciclovir by first-pass deacetylation and oxidation |
| Distribution | widely distributed; intracellular penciclovir triphosphate half-life 7–20 hours in infected cells |
| Metabolism | rapidly deacetylated and oxidized to penciclovir in intestinal mucosa and liver |
| Excretion | renal (73% as penciclovir) |
| Half-life | 2–3 hours (penciclovir plasma); intracellular half-life much longer |
| Onset | rapid |
| Peak | 0.9 hours |
| Duration | 8–12 hours (dosing intervals) |
| Protein Binding | 20% |
| Vd | moderate |
Drug Interactions
| Drug / Agent | Mechanism | Severity |
|---|---|---|
| probenecid | reduces renal excretion of penciclovir; increased exposure | minor |
| other nephrotoxic drugs | additive nephrotoxicity potential | moderate |
Nursing Considerations
- Administer with or without food; initiate therapy promptly after symptom onset — within 72 hours for shingles, 24–48 hours for genital herpes for optimal efficacy.
- Reduce dose in renal impairment; monitor renal function with prolonged therapy.
- Counsel patients with genital herpes on safe sexual practices and that suppressive therapy reduces but does not eliminate shedding.
- Older patients with shingles should be counseled that antiviral therapy reduces both duration and severity of post-herpetic neuralgia if started promptly.
Clinical Pearls
- Penciclovir triphosphate (the active form of famciclovir) has a markedly longer intracellular half-life (7–20 hours vs. 1 hour for acyclovir triphosphate), which is the pharmacological basis for famciclovir's more convenient dosing intervals.
- For single-day treatment of cold sores (herpes labialis), famciclovir 1500 mg single dose has shown efficacy, offering a convenient option for patients.
Safety Profile
Pregnancy generally-safe
Lactation use-with-caution
Renal Adjustment Required
Hepatic Adjustment Not required
TDM Not required
Concordance Terms
Cross-referenced clinical concepts — click any term to see all content where it appears.