Aciclovir or Acyclovir is a commonly used anti-viral drug. It is most commonly used to treat herpes, Epstein-Barr virus and varicella. It is also used in treating HIV, but is of limited effectiveness. The drug was developed in the 1980s and was a breakthrough in viral treatment as it was both more effective against the viruses it is used against and has fewer side effects than the drugs it replaced. Most notably, it is not very toxic at therapeutic doses.