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# Drug Information Request: Acyclovir ## Overview Acyclovir is a synthetic purine nucleoside analogue that inhibits viral DNA polymerase, preventing DNA synthesis in susceptible herpesviruses. It requires activation by thymidine kinase to its monophosphate form. ## Primary Indications * Herpes simplex virus (HSV-1, HSV-2) infections (mucocutaneous, genital, encephalitis). * Varicella-zoster virus (VZV) infections (varicella/chickenpox, herpes zoster/shingles). ## 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/Severe Infection:** 10 mg/kg IV q8h for 14–21 days. * **Maximum:** IV dose typically capped at 1 g per dose unless directed by specific institutional or infectious disease protocols. ## Pediatric Dosing * **Varicella (Children >2 years):** 20 mg/kg PO QID (max 800 mg/dose) for 5 days. * **Neonatal HSV (IV):** 20 mg/kg IV q8h for 14–21 days (dose based on postmenstrual age). * **Severe HSV/VZV (>3 months):** 10 mg/kg IV q8h. ## Dose Adjustments * **Renal Impairment:** Mandatory. Dose reduction and/or extension of the dosing interval is required for CrCl <50 mL/min. * **Hemodialysis:** Administer dose *after* dialysis session. ## Contraindications * Hypersensitivity to acyclovir or valacyclovir. ## Adverse Effects * **Common:** Nausea, vomiting, diarrhea, headache. * **Serious:** Nephrotoxicity (crystalline nephropathy)—risk increased with rapid IV infusion and dehydration; neurotoxicity (lethargy, tremors, confusion), specifically in patients with renal failure. ## Key Drug Interactions * **Nephrotoxic Agents:** Increased risk of nephrotoxicity when combined with aminoglycosides, NSAIDs, or amphotericin B. * **Probenecid:** Decreases renal clearance of acyclovir, increasing serum concentrations. * **Mycophenolate:** Increases serum concentrations of both agents, potentially increasing toxicity. ## Monitoring * **Renal Function:** Monitor serum creatinine and BUN regularly. * **Hydration:** Maintain adequate fluid intake; ensure patient is well-hydrated before and during IV infusion to prevent crystal formation. * **Neurological Status:** Monitor for signs of neurotoxicity, especially in elderly or renally impaired patients. ## Clinical Pearls * **IV Administration:** Must be infused slowly (usually over at least 1 hour) to minimize the risk of renal tubular injury. * **Bioavailability:** PO bioavailability is low (15–30%); valacyclovir (the valine ester prodrug) is preferred for oral therapy due to significantly higher bioavailability. * **Timing:** Therapy should be initiated as soon as possible after symptom onset (ideally within 24–48 hours) for optimal efficacy. * **Local Protocols:** Always verify dosing against current institutional antibiograms and infectious disease guidelines, especially for severe or multidrug-resistant infections. *** *Disclaimer: This information is for educational purposes and does not replace professional clinical judgment. Always verify current prescribing information, institutional guidelines, and patient-specific factors before administering any medication.*