Acyclovir is a drug primarily used in the treatment of conditions caused by the herpes simplex virus. However, it is also sometimes used in the treatment of the Epstein-Barr virus. The drug works by interfering in the process of viral DNA synthesis, reducing the overall viral load present in the body. Thus, the drug does not cure the viral infection but does reduce the rate of viral reproductive success. This helps the body battle the infection and reduce overall treatment duration. In moderate doses, acyclovir is well tolerated though at higher doses it may cause side effects.