Famciclovir - Oral

What Is Famciclovir?

Famciclovir is one of several types of antiviral drugs used to treat various types of herpes infections in adults. The oral medication works by blocking the replication of the virus at the DNA level.

Famciclovir doesn’t “cure” herpes in the way that antibiotics kill bacterial infections. Still, it can quickly control the infection, speeding healing and reducing the risk of passing the virus to others. Famciclovir can be taken daily at a low dose for people with recurrent infections to reduce the frequency and duration of outbreaks.

It is only available by prescription and comes as a round or oval white tablet. It used to be available under the brand Famvir, which has since been discontinued. However, it is still available as a generic drug.

Drug Facts

Generic Name: Famciclovir 

Brand Name: Famvir (discontinued)

Drug Availability: Prescription

Administration Route: Oral

Therapeutic Classification: Antiviral

Available Generically: Yes

Controlled Substance: N/A

Active Ingredient: Famciclovir

Dosage Form: Tablet

What Is Famciclovir Used for?

Herpesvirus is a family of over 100 different viruses, eight of which can infect humans. Famciclovir is used to treat three of the most common herpesviruses:

  • Herpes simplex virus type 1 (HSV-1), the type most commonly associated with cold sores
  • Herpes simplex virus type 2 (HSV-2), the type most commonly associated with genital herpes
  • Herpes zoster, also known as shingles, caused by the reactivation of the chickenpox virus

Taking Famciclovir does not cure herpes or prevent you from passing herpes to others. If you have genital herpes, use a latex condom or avoid sexual intercourse, so you don’t accidentally spread the virus. Also, avoid touching an affected area or exposing others to contact with an infected area. Wash your hands frequently.

Famciclovir has specific indications for use. Depending on the type of infection, famciclovir may be used to treat an initial outbreak, recurrent outbreaks, or as a daily suppressive therapy to prevent a recurrence. The dosage and duration of treatment will vary based on these uses.

How to Take Famciclovir

Famciclovir is available as either a round 125-milligram (mg) or 250-milligram tablet or an oval 500-milligram tablet. The tablet can be taken with or without food.

Famciclovir is taken at the first signs of a cold sore or genital herpes, ideally within 48 hours of the appearance of symptoms. The treatment course can run anywhere from one to 10 days, depending on whether this is your first outbreak or a subsequent one.

For shingles, famciclovir should be taken within 72 hours of the appearance of symptoms. Doing so may reduce the severity or duration of the outbreak. After 72 hours, the benefits of treatment may be less.

When used as suppressive therapy, famciclovir is taken twice every day for up to one year. It may be taken longer, but studies are limited as to the effectiveness and safety of the drug beyond one year.


Famciclovir tablets can be stored safely at room temperature (around 77 degrees F). If traveling, it is OK to temporarily expose the drug to temperatures as low as 59 F and as high as 86 F.

Keep the medicines in a cool, dry drawer or cabinet away from direct sunlight or excessive heat. Always check the expiration date, and do not take expired drugs.

Off-Label Uses

Famciclovir is sometimes used off-label in the treatment of chronic hepatitis B virus (HBV) infection. There is some evidence that the long-term use of famciclovir may reduce the level of HBV in the bloodstream (called the viral load) and, by doing so, help slow disease progression.

Although famciclovir is commonly used to treat initial outbreaks of genital herpes, it is not formally approved for such use by the Food and Drug Administration (FDA). Even so, the Centers for Disease Control and Prevention (CDC) recommends famciclovir as one of three antiviral options for treating an initial herpes outbreak.

Additionally, while famciclovir is only approved to treat adults with HSV, clinical studies have looked at off-label pediatric dosing for this use.

How Long Does Famciclovir Take to Work?

If treated early, symptoms related to cold sores or genital herpes usually improve within two to three days. It may take longer to work for shingles.

What Are the Side Effects of Famciclovir?

This is not a complete list of side effects, and others may occur. A healthcare provider can advise you on side effects. If you experience other effects, contact your pharmacist or a healthcare provider. You may report side effects to the FDA at fda.gov/medwatch or 800-FDA-1088.

As with all drugs, famciclovir may cause side effects. Most are mild and will resolve on their own without treatment. On occasion, famciclovir has been known to cause serious side effects, often in people with underlying kidney problems.

Common Side Effects

Famciclovir is a prodrug, meaning a drug that is converted by the body into another drug. In this case, it is converted into penciclovir, which has a longer duration of action and therefore requires smaller or fewer doses. Because of this, famciclovir tends to have relatively few serious side effects.

Common side effects of famciclovir include:

  • Headache
  • Nausea
  • Diarrhea
  • Vomiting
  • Fatigue
  • Abdominal pain
  • Flatulence
  • Itching
  • Paresthesia (burning or pins-and-needles sensations)

Headache and nausea are the most common of these side effects.

Side effects are dose-dependent, meaning that the risk increases with the amount of drug that you take. This is why side effects are more common if you are on suppressive therapy (due to prolonged exposure to the drug) or being treated for shingles (due to higher doses).

Severe Side Effects

Call your healthcare provider immediately if you have serious side effects while taking famciclovir. Call 911 if your symptoms feel life-threatening or if you think you’re having a medical emergency.

Famciclovir is mainly excreted through the kidneys in urine. If your kidneys don’t work as well as they should, the drug may accumulate and form crystals in the inner tubules of the kidneys. If the kidney function is significantly impaired, this can lead to acute kidney failure in which the kidneys suddenly cannot filter the blood. However, kidney injury associated with famciclovir was only reported in people with inappropriately higher doses for their kidney function. At appropriate amounts, there is minimal risk. Dose reductions may be required for those with decreased kidney function.

Symptoms of acute kidney failure develop rapidly over hours or days and typically include:

In severe cases, acute kidney failure can lead to seizures, coma, and even death.

Drug Hypersensitivity

Famciclovir has been linked to a potentially life-threatening hypersensitivity reaction known as Stevens-Johnson syndrome (SJS) in extremely rare cases. SJS is more commonly associated with the long-term use of a drug.

Symptoms of SJS can develop over days or weeks and may include:

  • Unexplained widespread skin pain
  • A spreading red or purplish rash
  • Blistering lesions on the skin and in and around your mouth, nose, eyes, and genitals
  • Peeling of skin, often in large patches, within days of the blisters forming

These symptoms typically follow fever, burning eyes, fatigue, and a sore throat, usually one to three days before a rash develops.

Long-Term Side Effects

Famciclovir has been studied in people who have used the drug for up to one year. It is generally considered safe with a low risk of toxicity. This does not suggest that the same will necessarily be true if famciclovir is used for more extended periods. Some studies suggest that long-term use may lead to liver toxicity, although it is typically low grade and does not produce symptoms.

Speak with your healthcare provider about the benefits and potential risks of famciclovir if ongoing suppressive therapy is needed.

Report Side Effects

Famciclovir may cause other side effects. Call your healthcare provider if you have any unusual problems while taking this medication.

If you experience a serious side effect, you or your healthcare provider may send a report to the FDA's MedWatch Adverse Event Reporting Program or by phone (800-332-1088).

Dosage: How Much Famciclovir Should I Take?

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The dose of this medicine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.

The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.

  • For oral dosage form (tablets):
    • For treatment of shingles:
      • Adults—500 milligrams (mg) every eight hours for seven days.
      • Children—Use and dose must be determined by your doctor.
    • For suppression of recurrent genital herpes:
      • Adults—250 milligrams (mg) two times a day for up to one year.
      • Children—Use and dose must be determined by your doctor.
    • For treatment of recurrent genital herpes:
      • Adults—1000 milligrams (mg) two times a day for one day.
      • Children—Use and dose must be determined by your doctor.
    • For treatment of recurrent herpes labialis (cold sores):
      • Adults—1500 milligrams (mg) as a single dose.
      • Children—Use and dose must be determined by your doctor.
    • For treatment of recurrent herpes infections in HIV-infected patients:
      • Adults—500 milligrams (mg) two times a day for seven days.
      • Children—Use and dose must be determined by your doctor.


Due to the possible effects of this medication, there may be changes to how it is used. Your healthcare provider may adjust your dose of famciclovir if you have the following.

Kidney Disease

The famciclovir dose will need to be adjusted if you have moderate to severe kidney impairment. The exact dosage is based on a urine test known as creatinine clearance (CrCl) that measures kidney function. Typically, an adjustment would be needed if the CrCL falls below 60 milliliters per minute (mL/min). This normally involves a lower dose but may also include more frequent doses. People on hemodialysis who require famciclovir are also given a lower dose but only after the dialysis procedure.

Human Immunodeficiency Virus (HIV)

On the other hand, people with HIV may need an increased dosage. HIV causes immunosuppression, which makes it harder to fight infection. While people with normally functioning immune systems may find relief with a single 1,000- to 1,500-milligram dose, those with HIV may be advised to take 500 milligrams three times daily for seven days.

Missed Dose

If you miss a dose of famciclovir, take it as soon as you remember. If it is almost time for your next dose, skip the original dose and continue as normal. Never double up doses.

Overdose: What Happens If I Take Too Much Famciclovir?

An unintentional overdose of herpes antivirals is rare. Because the drugs are excreted from the body relatively quickly, they are unlikely to cause toxicity if you accidentally double up doses. If symptoms of an overdose occur, it will most likely be in people with advanced chronic kidney disease and those on dialysis.

Symptoms of a famciclovir overdose include:

  • Constipation
  • Diarrhea
  • Dizziness
  • Fatigue
  • Fever
  • Headache
  • Nausea
  • Vomiting

It is unknown at what dose famciclovir can become harmful, given that so few cases have been reported. If an intentional overdose were to occur, healthcare providers would treat the symptoms and, if needed, remove the excess drug from the bloodstream with hemodialysis. Keep famciclovir out of the reach of children to avoid accidental overdose.

What Happens If I Overdose on Famciclovir?

If you think you or someone else may have overdosed on famciclovir, call a healthcare provider or the Poison Control Center (800-222-1222).

If someone collapses or isn't breathing after taking famciclovir, call 911 immediately.


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If your symptoms do not improve within a few days, or if they become worse, check with your doctor.

The areas affected by herpes should be kept as clean and dry as possible. Also, wear loose-fitting clothing to avoid irritating the sores (blisters).

This medicine may cause some people to become dizzy, drowsy, or less alert than they are normally. If any of these side effects occur, do not drive, use machines, or do anything else that could be dangerous if you are dizzy or are not alert while you are taking famciclovir.

This medicine does not prevent the sexual transmission of genital herpes. You should avoid having sex when lesions are present to avoid infecting your partner.

What Are Reasons I Shouldn’t Take Famciclovir?

Famciclovir should never be taken by anyone with a known hypersensitivity to famciclovir or any other components of the drug. The same applies if you have had a severe reaction to Denavir (penciclovir cream).

There is limited data on the safety of famciclovir during pregnancy. However, the drug has not been associated with major birth defects or miscarriages. Studies in rats have shown no evidence of any fetal harm when the active drug (penciclovir) is given at comparably higher doses than those used in humans.

On the other hand, animal studies have shown that traces of penciclovir may be found in breast milk. It is unclear if this level of exposure poses any harm to an infant.

If you are pregnant, planning to get pregnant, or are breastfeeding, speak with your healthcare provider to fully understand the treatment’s benefits and potential risks.

What Other Medications Interact With Famciclovir?

There are few significant drug interactions associated with famciclovir. Those that may occur don’t appear to require a dose adjustment for either drug.

However, famciclovir can diminish the effect of the herpes zoster vaccine and the varicella virus vaccine. Famciclovir should only be given 14 days after vaccine administration.

What Medications Are Similar?

There are two other antivirals available by prescription that are also used to treat cold sores, genital herpes, and shingles:

Valtrex has a longer duration of action than Zovirax. It also has a higher bioavailability (i.e., the proportion of a drug entering the bloodstream) than Zovirax.

However, famciclovir is the ideal candidate for the treatment of shingles, a disease caused by the reactivation of the chickenpox virus (varicella zoster virus, or VSV).

With that said, Valtrex and Zovirax have greater activity against VZV in children with chickenpox and are approved for use in treating children who experience a severe reaction to the virus.

Frequently Asked Questions

  • What is famciclovir used for?

    Famciclovir is an antiviral drug taken by mouth that is used to treat cold sores, genital herpes, and shingles. It can also be used on an ongoing basis to reduce the risk of genital herpes in people with recurrent outbreaks.

  • How does famciclovir work?

    Famciclovir is a prodrug that is converted by the body into the active drug penciclovir. Once penciclovir enters an infected cell, it blocks the enzyme responsible for copying viral DNA. Without the means to replicate, the virus can be brought under control and placed back into a latent (dormant) state.

  • Which antiviral is better: Famciclovir, Valtrex, or Zovirax?

    Each antiviral has its distinct benefits and limitations. Famciclovir, Valtrex, and Zovirax are all considered equally well suited to treat cold sores, genital herpes, and shingles when used as directed.

  • How long does it take for famciclovir to work?

    Famciclovir acts quickly if you take it soon after symptoms appear. If treated early, you can generally expect an improvement in cold sores or genital herpes within two to three days; this may take longer for shingles. Herpesvirus outbreaks are far more challenging to treat if antiviral therapy is delayed.

  • When is it too late to start Famvir?

    There is no clear-cut answer. Generally speaking, it is best to start treatment no later than 48 hours after the onset of symptoms. The drug may be beneficial up to 72 hours after symptoms, but its effectiveness is thought to lessen after this. As a rule of thumb, sooner is always better than later.

  • Can a herpes infection be cured?

    No. Once you are infected with the herpes simplex virus or varicella-zoster virus, it stays in your body forever. Although the immune system can clear the virus from other tissues in the body, the virus can persist and lie dormant in clusters of nerve cells called ganglia. After that, the latent virus can spontaneously reactivate and cause symptomatic infection.

How Can I Stay Healthy While Taking Famciclovir?

There is sometimes no rhyme or reason you’ll suddenly get a cold sore, genital herpes, or shingles. Certain factors may increase your risk, but having a risk factor doesn’t necessarily mean you’ll experience an outbreak.

With that said, cold sores and genital herpes often develop when we are worn out and our immune systems are low. Staying healthy, eating well, and getting enough rest can go a long way toward keeping your immune system strong if you are prone to recurrence. If you have HIV, taking antiretroviral therapy daily can help rebuild the immune system and may reduce your risk of an outbreak.

If you have recurrent genital herpes, it often helps to have antivirals like famciclovir on hand should symptoms develop. In this way, you can hop on treatment immediately rather than waiting for a prescription.

If you are over 50 years old, the best way to prevent shingles is to get the shingles vaccine called Shingrix. The vaccine is given in two separate doses and can reduce your risk of an outbreak by over 90%. Even if you’ve gotten the earlier shingles vaccine, called Zostavax, you should still get Shingrix. Prevention is always better than treatment.

Medical Disclaimer

Verywell Health's drug information is meant for educational purposes only and is not intended to replace medical advice, diagnosis, or treatment from a healthcare provider. Consult your healthcare provider before taking any new medication(s). IBM Watson Micromedex provides some of the drug content, as indicated on the page.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By James Myhre & Dennis Sifris, MD
Dennis Sifris, MD, is an HIV specialist and Medical Director of LifeSense Disease Management. James Myhre is an American journalist and HIV educator.