Overview
Acyclovir is an antiviral medication used to treat infections caused by herpes viruses, including herpes zoster (shingles), genital herpes, and chickenpox. It works by inhibiting viral DNA replication, reducing the severity and duration of outbreaks. The drug is available in oral, intravenous, and topical formulations for various clinical scenarios.
Mechanism of Action
Acyclovir is a synthetic nucleoside analogue that selectively inhibits viral DNA synthesis. It is phosphorylated by viral thymidine kinase to acyclovir monophosphate, then converted to acyclovir triphosphate by cellular enzymes. Acyclovir triphosphate competitively inhibits viral DNA polymerase and incorporates into viral DNA, causing chain termination.
Indications
- Acute treatment of herpes zoster (shingles)
- Treatment of initial and recurrent episodes of genital herpes
- Treatment of chickenpox (varicella)
- Management of herpes simplex infections in immunocompromised patients
- Probenecid: Increases acyclovir AUC and extends half-life. Nephrotoxic Agents: Increased risk of renal dysfunction (e.g., aminoglycosides, NSAIDs, cyclosporine).
Common Doses
- 200 mg capsules
- 400 mg capsules
- 800 mg capsules
- 200 mg/5mL suspension
Dosage
Dosage varies by indication: Herpes zoster: 800 mg orally every 4 hours (5 times daily) for 7-10 days. Initial genital herpes: 200 mg every 4 hours (5 times daily) for 10 days. Chronic suppression: 400 mg twice daily for up to 12 months. Chickenpox (adults/children >40 kg): 800 mg 4 times daily for 5 days. Children (2+ years): 20 mg/kg per dose 4 times daily for 5 days. Renal impairment requires dosage adjustment.
Contraindications
- Hypersensitivity to acyclovir, valacyclovir, or any component of the formulation
- History of severe cutaneous adverse reactions (SCARs) to acyclovir or valacyclovir
Side Effects
- Common: nausea, vomiting, diarrhea, headache, malaise
- Serious: renal failure, neurological symptoms (confusion, hallucinations, seizures), severe cutaneous reactions
- Hematologic: anemia, thrombocytopenia, leukopenia
- Hepatic: elevated liver enzymes, hepatitis
- Neurological: dizziness, encephalopathy (especially in elderly/renal impairment)
- Common (>10%): Malaise (11.5%), Nausea (2–5%), Headache (2–12%).
Interactions
- Probenecid may decrease renal clearance of acyclovir, increasing plasma concentrations
- Nephrotoxic drugs (aminoglycosides, cyclosporine) may increase risk of renal toxicity when combined with acyclovir
Counseling Points
- Take as prescribed, even if symptoms improve
- Maintain adequate hydration to prevent renal complications
- Start treatment at earliest signs of outbreak for best results
- Avoid sexual contact during genital herpes outbreaks
- Report any signs of allergic reaction or severe skin rash immediately
- Use sun protection as photosensitivity reactions may occur