Aciclovir drug. Molecular model of the antiviral drug aciclovir (hydroxyethoxymethyl guanine). Atoms (spheres) are colour-coded: carbon (yellow), hydrogen (white), nitrogen (blue) and oxygen (red). Aciclovir is active against alpha herpes viruses and is used to treat herpes simplex and varicella-zoster infections. It works by inhibit- ing the production of viral DNA (deoxyribonucleic acid), which prevents the virus from reproducing.