Valtrex (valacyclovir) vs. Zovirax (acyclovir): What's the difference?

What is Valtrex? What is Zovirax?

Valtrex (valacyclovir) is a type of antiviral drug called a nucleoside analog used to treat infections with shingles (herpes zoster), genital herpes (herpes simplex genitalis), and cold sores (herpes labialis). Nucleoside analogs mimic one of the building blocks of DNA and they stop the spread of the herpes virus by preventing the viral DNA replication that is necessary for viruses to multiply. Acyclovir (Zovirax) and famciclovir (Famvir) are also nucleoside analogs. Valacyclovir is called a "prodrug," in that it is not active itself but rather, it is converted to acyclovir in the body, and it is the acyclovir that is active against the viruses. (Acyclovir itself is available as a topical, oral, and intravenous medication.) Valtrex thus is active against the same viruses as acyclovir, but Valtrex has a longer duration of action than acyclovir and can be taken fewer times each day.

Zovirax (acyclovir) is an antiviral drug, a synthetic nucleoside analogue, that has inhibitory activity (interferes with viral replication) against the herpes viruses, including herpes simplex 1 and 2 (cold sores and genital herpes), varicella-zoster (shingles and chickenpox), and Epstein-Barr virus (mononucleosis). Viruses take over living cells and reproduce themselves, often at the expense of the host cell. Zovirax is converted to an active form by the virus itself, and the virus then uses the active form of acyclovir rather than the nucleoside it normally uses to manufacture DNA, a critical component of viral replication. Incorporation of active acyclovir into new viral DNA stops the production of the DNA. Virally infected cells absorb more Zovirax than normal cells and convert more of it to the active form, which prolongs its antiviral activity.

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What are the side effects of Valtrex and Zovirax?

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:

Zovirax

The most common side effects are

Other reported side effects include:

  • agitation,
  • confusion,
  • rash,
  • anemia, and
  • muscle pain,
  • hypersensitivity reactions,
  • seizures,
  • agitation,
  • confusion,
  • anemia,
  • hepatitis, and
  • muscle pain.

What is the dosage for Valtrex vs. Zovirax?

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.

Zovirax

Acyclovir may be taken with or without food.

  • Adult oral doses for treating genital herpes are 200 mg every 4 hours (5 times daily) for 7-10 days or 400 mg three times daily for 5-10 days.
  • Herpes Zoster (shingles) is treated with 800 mg every 4 hours (5 times daily) for 7 to 10 days.
  • The dose for treating chicken pox is 800 mg 4 times daily. The usual adult intravenous dose is 5-10 mg/kg every 8 hours for 7-10 days.

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What supplements interact with Valtrex and Zovirax?

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.

Zovirax

Acyclovir may decrease levels of phenytoin (Dilantin) or valproic acid (Depakote, Depakote ER). Probenecid (Benemid) may increase acyclovir serum levels by decreasing renal excretion of acyclovir. Acyclovir may increase serum levels of theophylline (Theo-Dur, Respbid, Slo-Bid, Theo-24, Theolair, Uniphyl, Slo-Phyllin).

Combining acyclovir with cidofovir (Vistide), amphotericin B (Fungizone) or other drugs that reduce kidney function may increase harmful effects on the kidney.

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Are Valtrex and Zovirax 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.

Zovirax

There are no adequate studies of acyclovir in pregnant women. In a patient registry of women who used acyclovir during the first trimester, the rate of birth defects was similar to the rate of birth defects in the general population.

Acyclovir is excreted in breast milk, and a significant amount may be transferred to the infant.

Summary

Valtrex (valacyclovir) and Zovirax (acyclovir) are antiviral drugs used to treat infections with shingles (herpes zoster), genital herpes (herpes simplex genitalis), and cold sores (herpes labialis). Zovirax is also used to treat Epstein-Barr virus (mononucleosis). Side effects of Valtrex and Zovirax that are similar include headache, nausea, vomiting, diarrhea, and rash.

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Medically Reviewed on 7/23/2019
References
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