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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.
## Primary Indications
* HSV-1 and HSV-2 infections (genital, mucocutaneous, encephalitis).
* Varicella-zoster (chickenpox and shingles).
* Prophylaxis in immunocompromised patients.
## Adult Dosing
* **Genital Herpes (Initial):** 400 mg PO TID for 7–10 days.
* **Herpes Zoster (Shingles):** 800 mg PO 5 times daily for 7 days.
* **HSV Encephalitis:** 10 mg/kg IV q8h for 14–21 days.
* **Prophylaxis (Immunocompromised):** 400–800 mg PO BID.
## Pediatric Dosing
* **Neonatal HSV (Birth to 3 mo):** 20 mg/kg IV q8h for 14–21 days.
* **Varicella (Children >2yr, immunocompromised):** 20 mg/kg PO QID (max 800 mg/dose) for 5 days.
* **HSV Infection (Children ≥3 mo):** 10 mg/kg IV q8h or 20 mg/kg PO QID.
## Dose Adjustments
* **Renal Impairment:** Requires dose reduction and extended intervals based on creatinine clearance (CrCl).
* **Hemodialysis:** Administer dose *after* dialysis; supplemental doses may be required based on clinical assessment.
## Contraindications
* Hypersensitivity to acyclovir or valacyclovir.
## Adverse Effects
* **Common:** Nausea, vomiting, headache, dizziness.
* **Serious:** Crystalline nephropathy (risk increases with rapid IV infusion/dehydration), neurotoxicity (lethargy, tremors, confusion), and thrombotic thrombocytopenic purpura (TTP)/hemolytic uremic syndrome (HUS) in immunocompromised patients.
## Key Drug Interactions
* **Nephrotoxic Agents:** Increased risk of renal impairment (e.g., aminoglycosides, NSAIDs, cyclosporine).
* **Probenecid:** Decreases renal clearance of acyclovir, increasing serum concentrations.
* **Zidovudine:** Potential for extreme lethargy or neurotoxicity.
## Monitoring
* **Renal Function:** Monitor serum creatinine and BUN regularly during IV therapy.
* **Hydration:** Maintain adequate urine output to prevent crystal precipitation in renal tubules.
* **CNS Status:** Monitor for signs of neurotoxicity, especially in elderly patients or those with renal impairment.
## Clinical Pearls
* **IV Administration:** Always infuse over at least 1 hour to prevent crystal nephropathy. Rapid bolus is strictly contraindicated.
* **Hydration:** Ensure patients are well-hydrated during IV therapy.
* **Valacyclovir comparison:** Valacyclovir is a prodrug of acyclovir with better oral bioavailability; it is preferred for oral dosing to improve patient compliance.
* **Timing:** Efficacy is maximal when therapy is initiated within 48–72 hours of onset of symptoms/rash.
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*Disclaimer: This information is for educational purposes only. Clinical practice and therapeutic protocols may vary by institution. Always verify dosages and contraindications against the latest prescribing information or your local formulary before administration.*
