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# Drug Information Request: Acyclovir
## Overview
Acyclovir is a synthetic purine nucleoside analogue that inhibits herpes simplex virus (HSV) types 1 and 2, varicella-zoster virus (VZV), Epstein-Barr virus (EBV), and cytomegalovirus (CMV). It requires activation by viral thymidine kinase to its monophosphate form, subsequently being converted to the active triphosphate form, which inhibits viral DNA polymerase.
## Primary Indications
* HSV-1 and HSV-2 infections (mucocutaneous, genital, and encephalitis).
* Varicella-zoster virus (chickenpox and shingles).
## Adult Dosing
* **Genital Herpes (Initial):** 400 mg PO TID for 7–10 days.
* **Genital Herpes (Recurrent):** 400 mg PO TID for 5 days or 800 mg PO BID for 5 days.
* **Herpes Zoster (Shingles):** 800 mg PO 5 times daily for 7–10 days.
* **HSV Encephalitis:** 10 mg/kg IV every 8 hours for 14–21 days.
## Pediatric Dosing
* **Neonatal HSV:** 20 mg/kg IV every 8 hours for 14–21 days.
* **Varicella (Children >2 years):** 20 mg/kg/dose PO QID (max 800 mg/dose) for 5 days.
* **Immunocompromised HSV:** 10 mg/kg IV every 8 hours.
## Dose Adjustments
Renal impairment requires significant dosage reduction and/or interval extension. Patients with a CrCl < 50 mL/min necessitate adjustment based on institution-specific nomograms. Ensure adequate hydration to prevent crystalline nephropathy.
## Contraindications
* Hypersensitivity to acyclovir or valacyclovir.
## Adverse Effects
* **Common:** Nausea, vomiting, diarrhea, headache.
* **Severe:** Crystalline nephropathy (IV administration), neurotoxicity (lethargy, confusion, tremors, especially in renal impairment), and injection site reactions (phlebitis).
## Key Drug Interactions
* **Nephrotoxic Agents:** Concurrent use with aminoglycosides, NSAIDs, or amphotericin B increases the risk of renal failure.
* **Probenecid:** Decreases renal clearance of acyclovir, increasing serum concentrations and potential toxicity.
## Monitoring
* Serum creatinine and BUN (baseline and during therapy).
* Urine output and hydration status.
* Neurological status (in patients with renal impairment).
## Clinical Pearls
* **Hydration:** Always keep patients well-hydrated during IV infusion to prevent the drug from precipitating in the renal tubules. Administer IV doses as a slow infusion over at least 1 hour.
* **Conversion:** Oral bioavailability is low (15–30%); consider these differences when converting from IV to PO.
* **Timing:** Therapy is most effective when initiated during the prodromal phase or within 48–72 hours of rash onset.
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*Disclaimer: This information is for educational purposes only. Clinical practice protocols vary. Always verify current prescribing information, institutional guidelines, and patient-specific factors via reliable clinical databases (e.g., Lexicomp, UpToDate) before prescribing or administering medication.*
