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Famciclovir Side Effects

Medically reviewed by Last updated on Nov 13, 2021.

In Summary

Commonly reported side effects of famciclovir include: headache. See below for a comprehensive list of adverse effects.

For the Consumer

Applies to famciclovir: oral tablet

Side effects requiring immediate medical attention

Along with its needed effects, famciclovir may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking famciclovir:

Incidence not known

  • Black, tarry stools
  • bleeding gums
  • blistering, peeling, or loosening of the skin
  • blood in urine or stools
  • chills
  • clay-colored stools
  • cough
  • dark urine
  • dizziness
  • fever
  • joint or muscle pain
  • loss of appetite
  • pinpoint red spots on the skin
  • red, irritated eyes
  • sore throat
  • sores, ulcers, or white spots in the mouth or on the lips
  • unpleasant breath odor
  • unusual bleeding or bruising
  • vomiting of blood
  • yellow eyes or skin

Side effects not requiring immediate medical attention

Some side effects of famciclovir may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

More common

Less common

  • Bloated, full feeling
  • burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feeling
  • confusion as to time, place, or person
  • excess air or gas in the stomach or intestines
  • hives or welts
  • holding false beliefs that cannot be changed by fact
  • itching skin
  • mood or mental changes
  • passing gas
  • rash
  • redness of skin
  • seeing, hearing, or feeling things that are not there
  • unusual excitement, nervousness, or restlessness
  • unusual tiredness or weakness
  • vomiting

Incidence not known

  • Sleepiness or unusual drowsiness

For Healthcare Professionals

Applies to famciclovir: oral tablet


Famciclovir is generally well tolerated. The most common side effects reported in at least 1 indication by greater than 10% of patients are headache and nausea.[Ref]

Nervous system

Confusion and bradykinesia developed in an 80-year-old woman who received famciclovir for acute herpes zoster. The dosage administered was clinically appropriate and adjusted to her estimated creatinine clearance. Although herpes zoster may sometimes elicit neurological complications and cannot be ruled out as a contributing factor, the patient's mobility problems and mental deterioration corresponded temporally with drug administration (onset within 48 and 72 hours, respectively, of initiation and reinstitution of therapy).[Ref]

Very common (10% or more): Headache (up to 39.3%)

Common (1% to 10%): Migraine (up to 3.1%), paresthesia (up to 2.6%)

Frequency not reported: Insomnia

Postmarketing reports: Dizziness, somnolence[Ref]


An isolated case of acute necrotic-hemorrhagic pancreatitis occurred in a kidney graft recipient who was treated with famciclovir for severe hepatitis B infection. The author could not identify any other etiologic factor for the pancreatitis, although some possible predisposing factors were noted, including the patient's liver insufficiency and renal dysfunction.[Ref]

Very common (10% or more): Nausea (up to 13%)

Common (1% to 10%): Diarrhea (up to 9%), abdominal pain (up to 7.9%), vomiting (up to 5%), elevated lipase (greater than 1.5 times normal range high [NRH]; 4.9%), flatulence (up to 4.8%), elevated amylase (greater than 1.5 times NRH; 1.5%)

Rare (less than 0.1%): Pancreatitis

Frequency not reported: Dyspepsia, anorexia[Ref]


Common (1% to 10%): Dysmenorrhea (up to 7.6%)[Ref]


Common (1% to 10%): Fatigue (up to 4.8%)

Frequency not reported: Injury, back pain, viral infection (including influenza, influenza-like symptoms, and symptoms of a cold), asthenia, fever[Ref]


Common (1% to 10%): Pruritus (up to 3.7%), rash (up to 3.3%)

Postmarketing reports: Urticaria, erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis, angioedema (e.g., face, eyelid, periorbital, and pharyngeal edema), leukocytoclastic vasculitis[Ref]


Common (1% to 10%): Neutropenia (less than 0.8 times normal range low [NRL]; 3.2%), leukopenia (less than 0.75 times NRL; 1.3%)

Uncommon (0.1% to 1%): Anemia (less than 0.8 times NRL; 0.1%)

Postmarketing reports: Thrombocytopenia[Ref]


Common (1% to 10%): Elevated ALT (greater than 2 times NRH; 3.2%), elevated AST (greater than 2 times NRH; 2.3%), elevated total bilirubin (greater than 1.5 times NRH; 1.9%)

Postmarketing reports: Cholestatic jaundice, abnormal liver function tests[Ref]


Uncommon (0.1% to 1%): Elevated serum creatinine (greater than 1.5 times NRH; 0.2%)

Frequency not reported: Acute renal failure[Ref]


Postmarketing reports: Palpitations


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


Frequency not reported: Upper respiratory tract infection, pharyngitis[Ref]


1. Sacks SL, Aoki FY, Martel AY, Shafran SD, Lassonde M "Clinic-initiated, twice-daily oral famciclovir for treatment of recurrent genital herpes: a randomized, double-blind, controlled trial." Clin Infect Dis 41 (2005): 1097-104

2. "Product Information. Famvir (famciclovir)." SmithKline Beecham, Philadelphia, PA.

3. Romanowski B, Aoki FY, Martel AY, Lavender EA, Parsons JE, Saltzman RL "Efficacy and safety of famciclovir for treating mucocutaneous herpes simplex infection in HIV-infected individuals." Aids 14 (2000): 1211-7

4. Aoki FY, Tyring S, Diaz-Mitoma F, Gross G, Gao J, Hamed K "Single-day, patient-initiated famciclovir therapy for recurrent genital herpes: a randomized, double-blind, placebo-controlled trial." Clin Infect Dis 42 (2006): 8-13

5. DiazMitoma F, Sibbald RG, Shafran SD, Boon R, Saltzman RL "Oral famciclovir for the suppression of recurrent genital herpes: A randomized controlled trial." JAMA 280 (1998): 887-92

6. Wald A "New therapies and prevention strategies for genital herpes." Clin Infect Dis 28 (1999): s4-13

7. Luber AD, Flaherty JF "Famciclovir for treatment of herpesvirus infections." Ann Pharmacother 30 (1996): 978-85

8. Dworkin RH, Johnson RW, Breuer J, et al. "Recommendations for the management of herpes zoster." Clin Infect Dis 44 Suppl 1 (2007): S1-S26

9. Gales BJ, Gales MA "Confusion and bradykinesia associated with famciclovir therapy for herpes zoster." Am J Health Syst Pharm 53 (1996): 1454,1456

10. Saltzman R, Jurewicz R, Boon R "Safety of famciclovir in patients with herpes zoster and genital herpes." Antimicrob Agents Chemother 38 (1994): 2454-7

11. Abudalu M, Tyring S, Koltun W, Bodsworth N, Hamed K "Single-day, patient-initiated famciclovir therapy versus three-day valacyclovir regimen for recurrent genital herpes: a randomized, double-blind, comparative trial." Clin Infect Dis 47 (2008): 651-8

12. Spruance SL, Bodsworth N, Resnick H, et al. "Single-dose, patient-initiated famciclovir: A randomized, double-blind, placebo-controlled trial for episodic treatment of herpes labialis." J Am Acad Dermatol 55 (2006): 47-53

13. Simpson D, Lyseng-Williamson KA "Famciclovir: a review of its use in herpes zoster and genital and orolabial herpes." Drugs 66 (2006): 2397-416

14. Tyring S, Belanger R, Bezwoda W, Ljungman P, Boon R, Saltzman RL "A randomized, double-blind trial of famciclovir versus acyclovir for the treatment of localized dermatomal herpes zoster in immunocompromised patients." Cancer Invest 19 (2001): 13-22

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Some side effects may not be reported. You may report them to the FDA.