acyclovir

Acyclovir is an antiviral medication used to treat infections caused by herpesviruses, including herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) and varicella-zoster virus (VZV). It is a nucleoside analogue of guanine that requires activation within infected cells by phosphorylation: first by the viral thymidine kinase to acyclovir monophosphate, then by cellular kinases to active acyclovir triphosphate. The active form inhibits viral DNA polymerase and acts as a chain terminator, reducing viral replication while having comparatively less effect on host DNA synthesis.

Acyclovir is indicated for mucocutaneous HSV infections (initial and recurrent), genital herpes, and suppression in frequent recurrences, as well as for VZV infections such as varicella and shingles. It is available in multiple forms, including oral tablets, oral suspension, intravenous infusion, topical cream for cold sores, and ophthalmic ointment for herpetic keratitis. Dosing and duration depend on the infection site and severity, patient age, and renal function.

Pharmacokinetics: oral bioavailability is limited (approximately 10–30%), and the drug distributes widely in body fluids and tissues. It is excreted primarily unchanged in the urine, with a half-life around 2.5–3 hours in individuals with normal renal function; dose adjustments are needed in renal impairment. Acyclovir crosses the placenta and is detectable in breast milk.

Safety and adverse effects: common effects include nausea, diarrhea, and headache. Intravenous use can be associated with nephrotoxicity and CNS effects, especially with inadequate hydration. Topical use may cause local irritation. It is generally considered safe in pregnancy (category varies by jurisdiction) and is excreted in breast milk; clinicians weigh benefits and risks for nursing mothers.

Resistance can develop, particularly in immunocompromised patients, often due to mutations in viral thymidine kinase or DNA polymerase. Interactions include increased levels with certain nephrotoxic agents such as probenecid, and proper hydration is advised during IV therapy.