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What is Famciclovir?

By D. Jeffress
Updated May 17, 2024
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Famciclovir is an oral antiviral drug that is usually prescribed to treat various forms of herpes. It is usually reserved for people with chickenpox or shingles, though doctors also prescribe famciclovir for patients who suffer from frequent oral or genital herpes outbreaks. Famciclovir works by invading the DNA of active viruses in the body, preventing them from replicating. The drug is marketed internationally under the brand name Famvir®, but it is also available in generic form in most countries.

Herpes cannot be cured, though taking famciclovir as suggested by a doctor can significantly decrease recovery time from an episode and help prevent frequent outbreaks. The drug can relieve itching and burning sensations and cause open lesions to dry and scab over quicker than they would naturally. Patients who have shingles or chickenpox often need to apply topical ointments in addition to taking famciclovir to shorten healing time, though they should always consult with their doctors or pharmacists first to prevent adverse drug reactions.

The medication is classified as a prodrug, meaning that it must be chemically altered in the body before it can effectively do its job. When a person takes the drug, it is converted into a compound called penciclovir that mimics the amino acid guanine. Penciclovir takes the place of guanine in viral DNA sequences, disrupting the genetic code and preventing viral agents from replicating.

Famciclovir comes in 125, 250, and 500 milligram (mg) tablets. Doctors determine specific dosing regimens based on their patients' conditions, ages, and medical histories. Many patients only need to take one or a few doses to get the drug's maximum effect: adult patients who have genital herpes or cold sore outbreaks are usually instructed to take 1,000 to 1,500 mg over the course of one day. The drug has also proven effective in suppressing recurring outbreaks of genital herpes when it is taken in 250 mg doses twice a day indefinitely. Adults with shingles or chickenpox typically need to take 500 mg doses every eight hours for about a week to find relief. Clinical studies have shown that famciclovir is safe for use by children, though dosing amounts are usually lowered for young patients.

Patients occasionally experience mild side effects when taking famciclovir. The most common reported symptoms include nausea, headache, fatigue, and diarrhea. Less commonly, a person can experience an allergic reaction that produces a skin rash and potentially serious airway constriction. The drug can rarely affect the levels of healthy white or red blood cells in the body, leading to severe illness that requires hospitalization. There are several alternative medications for herpes treatment, so a person who experiences any side effects should report them to his or her doctor to learn about other options.

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