What is Valacyclovir?

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  • Written By: wiseGEEK Writer
  • Edited By: O. Wallace
  • Last Modified Date: 20 September 2019
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Valacyclovir is the generic name for the brand-name drug Valtrex®. This is an antiviral medication very similar in nature to another medicine called acyclovir. Acyclovir is still preferred in certain instances, especially if people who have HIV need to take this medicine, but Valtrex® is often thought superior because it is more greatly bioavailable (or more of the drug hits the bloodstream), than acyclovir. Both medicines have similar treatment targets and are used to prevent or treat herpes simplex infections or as a supportive medicine during outbreaks of shingles (Herpes zoster). Other uses include prevention or treatment of Epstein-Barr virus or cytomegalovirus, with the latter particularly given to prevent this infection among transplant recipients.

People take valacyclovir by mouth and the amount they take may depend on age and the condition targeted. Amount of time the drug is taken can also differ based on condition. When people take Valtrex® as herpes simplex prophylaxis they could take it for several months or longer to prevent outbreaks from occurring. Treatment may not last longer than a year, though doctors and patients generally reassess and determine if daily treatment is still required. Some doctors lean toward prescribing it for daily use for an indeterminate period because it appears to lower herpes virus shedding, which the virus can do even when active outbreak is not present, presenting potential exposure to others.


There can be potential drug interactions between valacyclovir and other medications. People should review all prescribed drugs with their physicians to determine that no interaction exists. Some of the problem drugs in this area include cimetidine (Tagamet®), which is an antacid available over the counter in many places. This drug, and another called probenecid, can raise blood levels of valacyclovir, and consideration should be given as to whether this is wise.

Like many drugs, valacyclovir is associated with some side effects, though these tend to be mild and usually don’t affect more than one percent of the population of people who use this drug. Some of the more noted side effects can include some stomach upset such as nausea and vomiting. People also report headache with this medication. Though very rare, occasional anaphylactic allergic reaction has occurred from taking this medicine and even more rarely, dangerous skin disease like Stevens-Johnson syndrome have been reported. Development of unusual rash with or without face and tongue swelling and difficulty breathing is a sign to get immediate medical care.

It is not clear whether valacyclovir poses risk to the developing fetus or to nursing women and their babies. This is more due to lack of serious inquiry into the matter. Pregnant or nursing women should thus weigh risks versus benefits carefully with their doctors before using this drug. At present, there is little evidence in animal studies to suggest Valtrex® is of risk to fetuses or nursing babies, but such studies need to be replicated in the human environment before full conclusions can be drawn in this area.


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