Pharmacology: Acyclovir is a synthetic purine nucleoside analogue estructurally related to guanine. It is used mainly for the treatment of viral infections due to herpes simplex virus (type 1 and 2) and varicella-zoster virus (herpes zoster and chickenpox). Absorption of acyclovir is usually slight following topical application to intact skin. It is only partially absorbed from the gastrointestinal tract (about 15% to 30% of a dose given by mouth). The absorbed acyclovir is excreted through the kidney by both glomerular filtration and tubular secretion. Acyclovir crosses the placenta and is excreted in breast milk in concentration approximately 3 times higher than those in maternal serum.