Aciclovir or Acyclovir is a commonly used antiviral drug. It's most commonly used to treat herpes, Epstein-Barr virus, and varicella. It's also used in treating HIV, but is of limited effectiveness.
Acyclovir pills, courtesy Ragesoss, via Wikipedia
The drug was developed in the 1980s and was a breakthrough in viral treatment as it was both more effective against the viruses it's used against and has fewer side effects than the drugs it replaced. Most notably, it is not very toxic at therapeutic doses.
The drug works by attaching to strands of viral DNA, preventing them from growing. Previous drugs with the same action also attached to the DNA of normal cells, inhibiting their function. Aciclovir is more than 100 times more likely to attach to viral DNA than human DNA.
Although the drug may be taken orally, most of the drug will not pass into the bloodstream if taken in this manner. As such, it must be administered intravenously if the concentration of the drug is important.
Aciclovir is very safe, but should be avoided in pregnant women (although defects to the fetus are very rare) and people with kidney function issues.