Which antiviral is used intravenously for herpes simplex virus encephalitis?

• A. Valacyclovir
• B. Acyclovir (IV for encephalitis)
• C. Famciclovir
• D. Ganciclovir

Answer: (B)

Treat HSV encephalitis with high-dose intravenous acyclovir because rapid, high concentrations in the CSF are essential to halt viral replication and reduce mortality and neurologic damage. Acyclovir is activated preferentially in HSV-infected cells (via viral thymidine kinase) and then inhibits viral DNA polymerase, sparing uninfected cells. Oral antiviral prodrugs like valacyclovir or famciclovir can become acyclovir or penciclovir, but they don’t reliably achieve the necessary CNS levels quickly enough for a life-threatening CNS infection. Ganciclovir covers CMV more effectively and carries greater marrow toxicity; it’s not the preferred first-line agent for HSV encephalitis.