Antiviral is an agent that kills a virus or that suppresses its ability to replicate and, hence, inhibits its capability to multiply and reproduce. Antivirals include acyclovir, amantadine, famciclovir, ganciclovir, lamivudine, ribavirin, and valacyclovir. Action, uses, dosages and side effects of all these drugs are also covered in this chapter. Common side effects are nausea, vomiting, diarrhea, headache, vaginitis, moniliasis, phlebitis. Serious side effects are fatal metabolic encephalopathy, acute renal failure, and blood dyscrasias.