Nanclovir-400 Aciclovir 400 mg is an antiviral medication that is used to treat several types of the herpes virus. This medication works by reducing the growth and spread of the herpes virus. A 5-day course of Nanclovir-400 Aciclovir 400 mg will help to reduce the duration of genital herpes sores by a day or two. It will also reduce the severity of symptoms and reduce the level of infection. If this medicine is taken in time, it may even prevent sores from breaking open. Taking Nanclovir-400 Aciclovir 400 mg twice a day for 6 to 12 months can help to prevent outbreaks of the virus.