Valaciclovir (Valtrex) is an anti-viral drug. It is a pro-drug of aciclovir (Zovirax), meaning that it is broken down into aciclovir within the body. Valaciclovir produces higher blood levels of the active drug than is possible using the conventional tablet formulation of aciclovir.

Valaciclovir was formerly known by the codename BW256U87, and is known by the generic name netivudine. It is produced by GlaxoSmithKline.

In February 1995, an international study testing valaciclovir and high doses of aciclovir as preventive treatment for cytomegalovirus was stopped seven months early because participants who received valaciclovir were more likely to develop side-effects and had a significantly higher death rate, possibly due to higher levels of active drug. However, since valaciclovir was taken for a shorter period than aciclovir, there was no difference between the two arms once the full follow-up period of the study was completed. However, valaciclovir is effective in preventing cytomegalovirus in patients with HIV. A study of 310 patients with advanced HIV disease found that the drug lowered cytomegalovirus levels in the blood, delaying the onset of disease to a greater extent than aciclovir^[1][2].

Recent research has shown that valaciclovir treatment reduces the onward transmission of herpes simplex virus type 2 by 50%, and suppresses herpes simplex virus shedding from the genital tract^[3]. In HIV-positive people, a dose of 1000mg twice daily was shown to be comparable to 200mg aciclovir five times daily in treating a single episode of genital herpes, while 500mg twice daily was superior to placebo in preventing or delaying recurrences. The two drugs have similar safety profiles^[4][5].

Valaciclovir appears to be linked to a potentially fatal type of bleeding in the kidneys, known as haemolytic uraemic syndrome. In the United States, the drug labelling carries a warning against its use in people with immune suppression.