Nucleoside antiviral (penciclovir prodrug) Pregnancy: eMC §4.6: there is a limited amount of data (fewer than 300 pregnancy outcomes) from use in pregnant women; the cumulative analysis of prospective and retrospective cases did not provide evidence that famciclovir causes any specific foetal defect or congenital anomaly, and animal studies have shown no embryotoxic or teratogenic effects with famciclovir or penciclovir. Famciclovir should only be used during pregnancy when the potential benefits of treatment outweigh the potential risks. It is unknown whether famciclovir is excreted in human breast milk, but animal studies have shown excretion of penciclovir in milk; if the woman's condition mandates treatment, discontinuation of breast-feeding may be considered.

Famciclovir

Brand names: Famvir

Famciclovir is an oral antiviral prodrug used to treat herpes zoster (shingles) and herpes simplex infections, including genital herpes.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Herpes zoster and ophthalmic zoster in immunocompetent adults: 500 mg three times daily for seven days

Route: Oral - can be taken without regard to meals

Frequency: Three times daily

eMC §4.2 (Famciclovir 250 mg film-coated tablets). Treatment should be initiated as soon as possible after diagnosis of herpes zoster or ophthalmic zoster. OTHER ADULT REGIMENS FROM §4.2: herpes zoster in IMMUNOCOMPROMISED adults - 500 mg three times daily for ten days. First episode of genital herpes in immunocompetent adults - 250 mg three times daily for five days. Episodic treatment of recurrent genital herpes in immunocompetent adults - 125 mg twice daily for five days. Episodic treatment of recurrent genital herpes in immunocompromised adults - 500 mg twice daily for seven days. Suppression of recurrent genital herpes in immunocompetent adults - 250 mg twice daily. Suppression of recurrent genital herpes in immunocompromised adults - 500 mg twice daily. For episodic treatment, start as soon as possible after the onset of prodromal symptoms (tingling, itching, burning, pain) or lesions. SUPPRESSIVE THERAPY should be discontinued after a maximum of 12 months of continuous antiviral therapy to reassess recurrence frequency and severity; the minimum reassessment period should include two recurrences, and patients who continue to have significant disease may restart suppressive therapy. HEPATIC IMPAIRMENT: no dose adjustment in mild or moderate impairment; no data in severe impairment, in whom conversion of famciclovir to penciclovir may be impaired, giving lower penciclovir concentrations and possible loss of efficacy. ELDERLY (over 65 years): no dose modification unless renal function is impaired. ZOSTER SEVERITY (§4.4): consider intravenous antiviral therapy where the response to oral therapy is insufficient; patients with complicated herpes zoster (visceral involvement, disseminated zoster, motor neuropathies, encephalitis, cerebrovascular complications) should be treated intravenously, as should immunocompromised patients with ophthalmic zoster or a high risk of dissemination and visceral organ involvement. BLACK PATIENTS: a placebo-controlled study in immunocompetent Black patients with recurrent genital herpes showed no difference in efficacy between famciclovir 1000 mg twice daily for one day and placebo; this lack of efficacy of the one-day regimen cannot be extrapolated to the five-day 125 mg twice daily regimen or to other indications. PAEDIATRIC: safety and efficacy in children and adolescents under 18 years have not been established - verify any paediatric use against a children's formulary. US LABELLING DIFFERS and should be reconciled by the clinician: herpes labialis (cold sores) 1500 mg as a single dose; recurrent genital herpes 1000 mg twice daily for 1 day; suppressive therapy 250 mg twice daily; herpes zoster 500 mg every 8 hours for 7 days; recurrent orolabial or genital herpes in HIV-infected adults 500 mg twice daily for 7 days.

Dose adjustments

Renal

eMC §4.2: because reduced penciclovir clearance tracks reduced renal function, dose adjustment by creatinine clearance is required (Table 1). Herpes zoster in immunocompetent adults (nominal 500 mg three times daily for 7 days): CrCl 60 mL/min or more, unchanged; 40-59, 500 mg twice daily; 20-39, 500 mg once daily; under 20, 250 mg once daily; haemodialysis, 250 mg following each dialysis - all for 7 days. The same reductions apply to the 10-day immunocompromised zoster course. First episode of genital herpes (250 mg three times daily for 5 days): CrCl 40 or more, unchanged; 20-39, 250 mg twice daily; under 20, 250 mg once daily; haemodialysis, 250 mg following each dialysis. Episodic recurrent genital herpes in immunocompetent adults (125 mg twice daily for 5 days): CrCl 20 or more, unchanged; under 20, 125 mg once daily; haemodialysis, 125 mg following each dialysis. Episodic recurrent genital herpes in immunocompromised adults (500 mg twice daily for 7 days): CrCl 40 or more, unchanged; 20-39, 500 mg once daily; under 20, 250 mg once daily; haemodialysis, 250 mg following each dialysis. Suppression in immunocompetent adults (250 mg twice daily): CrCl 40 or more, unchanged; 20-39, 125 mg twice daily; under 20, 125 mg once daily; haemodialysis, 125 mg following each dialysis. Suppression in immunocompromised adults (500 mg twice daily): CrCl 40 or more, unchanged; 20-39, 500 mg once daily; under 20, 250 mg once daily; haemodialysis, 250 mg following each dialysis. Because 4 hours of haemodialysis reduces plasma penciclovir concentrations by up to 75%, famciclovir should be administered immediately following dialysis.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients (eMC §4.3)
Hypersensitivity to penciclovir (eMC §4.3). The US labelling additionally names hypersensitivity to Denavir (penciclovir cream)

Side effects

Headache - common (and, with nausea, the most frequently reported reaction in clinical studies; generally mild or moderate and at an incidence similar to placebo)
Nausea, vomiting, abdominal pain and diarrhoea - common
Dizziness (common); somnolence and confusional state predominantly in the elderly (uncommon); hallucinations (rare); seizure (post-marketing, frequency not known)
Rash and pruritus (common); angioedema including face, eyelid, periorbital and pharyngeal oedema, and urticaria (uncommon); serious skin reactions such as erythema multiforme, Stevens-Johnson syndrome and toxic epidermal necrolysis, and hypersensitivity vasculitis (post-marketing, frequency not known); anaphylactic shock and anaphylactic reaction (post-marketing)
Abnormal liver function tests (uncommon), cholestatic jaundice (rare), thrombocytopenia (uncommon) and palpitations (uncommon). In immunocompromised patients, nausea, vomiting and abnormal liver function tests were reported more frequently, especially at higher doses. Acute renal failure has been reported (US §5.1) in patients with underlying renal disease given inappropriately high doses for their level of renal function

Interactions

Probenecid (eMC §4.5) - concurrent use may increase plasma concentrations of penciclovir, the active metabolite, by competing for elimination. Patients receiving famciclovir 500 mg three times daily with probenecid should be monitored for toxicity; if severe dizziness, somnolence, confusion or other central nervous system disturbances occur, a dose reduction of famciclovir to 250 mg three times daily may be considered
Raloxifene (eMC §4.5) - famciclovir needs aldehyde oxidase to be converted into penciclovir, and raloxifene is a potent inhibitor of this enzyme in vitro; co-administration could affect penciclovir formation and hence efficacy, so the clinical efficacy of antiviral therapy should be monitored
No other clinically significant interactions have been identified (eMC §4.5). US §7 reports no clinically significant effect on digoxin, zidovudine, zidovudine glucuronide or emtricitabine, and no clinically significant alteration in penciclovir pharmacokinetics with allopurinol, cimetidine, theophylline, promethazine or antacids

Clinical monograph

How it works

It is converted to penciclovir, which is phosphorylated by viral thymidine kinase and then host kinases to a triphosphate that inhibits viral DNA polymerase and DNA replication.

Prescribing in practice

Treatment of herpes zoster is most effective when started as early as possible after rash onset, so prompt initiation is the priority.
Dose adjustment is required in renal impairment because penciclovir is renally cleared.
It does not eradicate latent virus, so recurrences can still occur after treatment.

Monitoring

Routine laboratory monitoring is not usually required, but assess renal function in those at risk and review clinical response.

Counselling the patient

Start treatment as soon as possible after symptoms or rash begin.
Maintain good fluid intake, particularly if you have kidney problems.
This treats outbreaks but does not cure the underlying infection, which may recur.

Evidence & guidelines

Famciclovir is an established oral option for herpes zoster and herpes simplex in UK clinical guidance, comparable to other guanine-analogue antivirals.

Reference: NICE CKS; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways