Famvir is an antiviral used in the treatment of shingles or in certain cases, genital herpes. When you take Famvir, it is absorbed into your bloodstream, and then converted into another drug called penciclovir. A virus infects the cells and begins to multiply over and over again, making new viruses. Penciclovir stops the herpes simplex virus types 1 and 2 from multiplying, which may reduce the symptoms of the outbreak and helps sores to heal faster.