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Acute renal failure is discussed in greater detail in other sections of the label [see WARNINGS AND PRECAUTIONS].

The most common adverse events reported in at least 1 indication by > 10% of adult patients treated with FAMVIR are headache and nausea.

Clinical Trials Experience in Adult Patients

Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared with rates in the clinical trials of another drug and may not reflect the rates observed in practice.

Immunocompetent patients: The safety of FAMVIR has been evaluated in active- and placebo-controlled clinical studies involving 816 FAMVIR-treated patients with herpes zoster (FAMVIR, 250 mg three times daily to 750 mg three times daily); 163 FAMVIR-treated patients with recurrent genital herpes (FAMVIR, 1000 mg twice daily); 1,197 patients with recurrent genital herpes treated with FAMVIR as suppressive therapy (125 mg once daily to 250 mg three times daily) of which 570 patients received FAMVIR (open-labeled and/or double-blind) for at least 10 months; and 447 FAMVIR-treated patients with herpes labialis (FAMVIR, 1500 mg once daily or 750 mg twice daily). Table 2 lists selected adverse events.

Table 2 : Selected Adverse Events (all grades and without regard to causality) Reported by ≥ 2% of Patients in Placebo-Controlled Famvir Trials*

Table 3 lists selected laboratory abnormalities in genital herpes suppression trials.

Table 3 : Selected Laboratory Abnormalities in Genital Herpes Suppression Studies*

HIV-infected patients: In HIV-infected patients, the most frequently reported adverse events for FAMVIR (500 mg twice daily; n=150) and acyclovir (400 mg, 5x/day; n=143), respectively, were headache (17% vs. 15%), nausea (11% vs. 13%), diarrhea (7% vs. 11%), vomiting (5% vs. 4%), fatigue (4% vs. 2%), and abdominal pain (3% vs. 6%).

Postmarketing Experience

The adverse events listed below have been reported during post-approval use of FAMVIR. Because these events are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure:

Blood and lymphatic system disorders: Thrombocytopenia

Hepatobiliary disorders: Abnormal liver function tests, cholestatic jaundice

Nervous system disorders: Dizziness, somnolence

Psychiatric disorders: Confusion (including delirium, disorientation, and confusional state occurring predominantly in the elderly), hallucinations

Skin and subcutaneous tissue disorders: Urticaria, erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis, angioedema (e.g. face, eyelid, periorbital, and pharyngeal edema.)

Potential for FAMVIR to Affect Other Drugs

The steady-state pharmacokinetics of digoxin were not altered by concomitant administration of multiple doses of famciclovir (500 mg three times daily). No clinically significant effect on the pharmacokinetics of zidovudine, its metabolite zidovudine glucuronide, or emtricitabine was observed following a single oral dose of 500 mg famciclovir coadministered with zidovudine or emtricitabine.

An in vitro study using human liver microsomes suggests that famciclovir is not an inhibitor of CYP3A4 enzymes.

Potential for Other Drugs to Affect Penciclovir

No clinically significant alterations in penciclovir pharmacokinetics were observed following single-dose administration of 500 mg famciclovir after pre-treatment with multiple doses of allopurinol, cimetidine, theophylline, zidovudine, promethazine, when given shortly after an antacid (magnesium and aluminum hydroxide), or concomitantly with emtricitabine. No clinically significant effect on penciclovir pharmacokinetics was observed following multiple-dose (three times daily) administration of famciclovir (500 mg) with multiple doses of digoxin.

Concurrent use with probenecid or other drugs significantly eliminated by active renal tubular secretion may result in increased plasma concentrations of penciclovir.

The conversion of 6-deoxy penciclovir to penciclovir is catalyzed by aldehyde oxidase. Interactions with other drugs metabolized by this enzyme and/or inhibiting this enzyme could potentially occur. Clinical interaction studies of famciclovir with cimetidine and promethazine, in vitro inhibitors of aldehyde oxidase, did not show relevant effects on the formation of penciclovir. Raloxifene, a potent aldehyde oxidase inhibitor in vitro, could decrease the formation of penciclovir. However, a clinical drug-drug interaction study to determine the magnitude of interaction between penciclovir and raloxifene has not been conducted.

Last reviewed on RxList: 2/16/2012
This monograph has been modified to include the generic and brand name in many instances.