Aciclovir powder, also known as acyclovir powder, is a sterile, white, crystalline, lyophilized powder used as a medication to treat herpes virus infections like genital herpes, chickenpox, and shingles; it is a synthetic nucleoside analog that works by inhibiting the replication of the herpes virus, and is typically administered as a solution prepared by reconstituting the powder with sterile water for injection before intravenous use.