Valtrex vs. Famvir: What's the difference?

What is Valtrex? What is Famvir?

Valtrex (valacyclovir) is an antiviral medication used to treat shingles (herpes zoster), genital herpes (herpes simplex genitalis), and cold sores (herpes labialis). Valtrex is a nucleoside analog that mimics a building block of DNA and stops the spread of herpes virus in the body by preventing the replication of viral DNA necessary for viruses to multiply. Other nucleoside analogs include acyclovir (Zovirax) and famciclovir (Famvir). Valacyclovir is a "prodrug," that in itself is not active. It is converted to acyclovir in the body and it is the acyclovir that is active against the viruses. (Acyclovir is available as a topical, oral, and intravenous medication.) Valacyclovir is active against the same viruses as acyclovir, but it has a longer duration of action than acyclovir and may be taken fewer times each day.

Famvir (famciclovir) is an antiviral drug active against the Herpes viruses, including herpes simplex 1 and 2 (cold sores and genital herpes) and varicella-zoster (shingles and chickenpox). It is a nucleoside analog that mimics one of the building blocks of DNA. It stops the spread of herpes virus in the body by preventing the replication of viral DNA that is necessary for viruses to multiply. Other nucleoside analogs include acyclovir (Zovirax) and valacyclovir (Valtrex). Famvir is actually a "prodrug," that is, not active directly against viruses. Instead, Famvir is converted to penciclovir in the body, and it is the penciclovir that is active against the viruses. Famvir is active against the same viruses as acyclovir but has a longer duration of action so it can be taken fewer times each day. Famvir does not cure or stop the spread of herpes infections. Famvir relieves pain, burning, itching, and tingling, and also heals and prevents sores associated with herpes infections.

What are the side effects of Valtrex and Famvir?

Valtrex

The side effect profile of Valtrex is similar to that of acyclovir (Zovirax).

Common side effects are:

Other important side effects are:

More serious side effects include central nervous system side effects which are more likely to happen in the elderly, for example:

Famvir

The most common side effects associated with the use of famciclovir are:

Other important side effects which are serious, but rare, include

  • serious allergic reactions,
  • serious skin reactions,
  • jaundice,
  • abnormal tests of liver function, and
  • reduced white blood cells (neutropenia) or platelets (thrombocytopenia).

Cases of kidney failure have been reported when higher than recommended doses of famciclovir were administered to patients with underlying kidney problems.

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What is the dosage for Valtrex vs. Famvir?

Valtrex

  • Valtrex may be taken with or without food.
  • In people with kidney disease, doses need to be reduced.
  • For the treatment of herpes zoster (shingles), the usual dose is 1 gm. three times a day for 7 days. Treatment should begin at the first symptom and is most effective if started within 48 hours of the onset of rash.
  • The dose for chickenpox is 20 mg/kg 3 times daily for 5 days (maximum dose is 1000 mg 3 times daily) and treatment should start at the earliest sign or symptom.
  • For the treatment of an initial episode of genital herpes, the usual dose is 1 gram (1000 mg) twice daily for 10 days. For the treatment of recurrent genital herpes, the usual dose is 500 mg twice daily for 3 days. For best results, treatment should be initiated within 12 hours of the start of symptoms.
  • The dose for cold sores is 2000 mg (2 grams) every 12 hours for 1 day.

Famvir

Famciclovir may be taken with or without food. The recommended doses are:

  • Recurrent genital herpes: 1,000 mg every 12 hours for one day.
  • Recurrent cold sores: 1,500 mg as a single dose.
  • Suppression of recurrent genital herpes: 250 mg twice daily.
  • Shingles: 500 mg every 8 hours for 7 days.
  • HIV-infected patients (cold sores or genital herpes): 500 mg twice daily for 7 days.

What drugs interact with Valtrex and Famvir?

Valtrex

Probenecid (Benemid) and cimetidine (Tagamet) may reduce the kidney's clearance of Valtrex, leading to higher concentrations in the blood. This may lead to side effects of Valtrex.

Famvir

Probenecid (Benemid) may reduce the kidney's removal of famciclovir leading to higher concentrations of famciclovir in the blood. This may lead to side effects from famciclovir.

Are Valtrex and Famvir safe to use while pregnant or breastfeeding?

Valtrex

Valtrex showed no effects on the fetus in animal studies; however, there has been no adequate evaluation of valacyclovir or (acyclovir) in pregnant women. The incidence of birth defects in women taking acyclovir is about the same as in the general population. Valtrex should only be used during pregnancy when the benefits to the mother outweigh risks to the fetus.

It is not known whether Valtrex is excreted into breast milk. It is known, however, that among women taking acyclovir, concentrations of acyclovir in breast milk are about four times higher than in the mother's blood. The safety of valacyclovir in breastfeeding infants has not been established. Methods other than breastfeeding should be considered if valacyclovir must be taken while nursing.

Famvir

Famciclovir has not been adequately studied in pregnant women.
It is not known if famciclovir is excreted into human breast milk. Women may wish to consider an alternative to breastfeeding if famciclovir is taken.

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Summary

Valtrex (valacyclovir) and Famvir (famciclovir) are antiviral drugs used to treat infections with shingles (herpes zoster), genital herpes (herpes simplex genitalis), and cold sores (herpes labialis). Side effects of Valtrex and Famvir that are similar include headache, nausea, vomiting, reduced white blood cells, diarrhea, and rash.

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Medically Reviewed on 7/23/2019
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