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Drug Reference

acyclovir

Antiviral· Brands: Zovirax, Acivir, ViraxIVPOTOP

Mechanism of action
Acyclovir is a nucleoside analogue with antiviral activity against herpes simplex virus types 1 and 2 and varicella-zoster virus. Inside infected cells, viral thymidine kinase phosphorylates acyclovir to acyclovir monophosphate, which host enzymes convert to acyclovir triphosphate. This triphosphate competitively inhibits viral DNA polymerase and is incorporated into viral DNA, causing chain termination. The high selectivity for infected cells results from the requirement for viral thymidine kinase for initial phosphorylation.
Pharmacokinetics
Oral bioavailability is about 15-30% and is not significantly affected by food. Acyclovir is widely distributed, including to cerebrospinal fluid, and is excreted primarily unchanged in urine by glomerular filtration and tubular secretion. Plasma half-life is approximately 2-3 hours with normal renal function, but is prolonged in renal impairment, necessitating dose reduction. Topical absorption is minimal.
Indications & dosing

Herpes simplex encephalitis

Treat for 14-21 days; adjust in renal impairment.

  • adult · IV: 10 mg/kg every 8 hours · Treat for 14-21 days; adjust in renal impairment.

Neonatal herpes simplex virus infection

Duration varies; expert advice recommended.

  • neonatal · IV: 20 mg/kg every 8 hours · Duration varies; expert advice recommended.

Varicella (chickenpox) in immunocompetent patients

Max 800 mg per dose; start within 24 hours of rash onset.

  • pediatric · PO: 20 mg/kg four times daily · Max 800 mg per dose; start within 24 hours of rash onset.

Herpes zoster (shingles)

Treat for 7 days; ideally within 72 hours of rash.

  • adult · PO: 800 mg five times daily · Treat for 7 days; ideally within 72 hours of rash.

First episode genital herpes

Treat for 7-10 days.

  • adult · PO: 400 mg three times daily · Treat for 7-10 days.

Recurrent genital herpes (episodic)

Treat for 5 days; ideally at prodrome.

  • adult · PO: 400 mg three times daily · Treat for 5 days; ideally at prodrome.

Suppression of recurrent genital herpes

Long-term; reassess need periodically.

  • adult · PO: 400 mg twice daily · Long-term; reassess need periodically.
Contraindications & precautions
  • Hypersensitivity to acyclovir or valacyclovir · Serious hypersensitivity reactions, including anaphylaxis and angioedema, have been reported.
  • Severe renal impairment (creatinine clearance < 10 ml/min) · Dose adjustment is required to avoid accumulation and toxicity.
  • Dehydration · Increased risk of nephrotoxicity, particularly with intravenous administration.
  • Pregnancy (especially first trimester) · Use only if the potential benefit justifies the potential risk to the fetus.
  • Ensure adequate hydration, especially during intravenous administration, to prevent crystalluria and nephrotoxicity.
  • Adjust dose in renal impairment and monitor renal function (serum creatinine) regularly.
  • Administer intravenous acyclovir by slow infusion over at least 1 hour to reduce the risk of renal damage and neurotoxicity.
  • Monitor for neurological symptoms (confusion, hallucinations) in elderly or renally impaired patients.
  • Use with caution in pregnancy and lactation.
Adverse effects
  • Nausea
  • Diarrhoea
  • Headache
  • Rash
  • Nephrotoxicity (acute kidney injury)serious
  • Neurotoxicity (confusion, hallucinations, tremors, seizures)serious
  • Leukopenia or thrombocytopeniaserious
  • Anaphylaxisserious
Renal & hepatic adjustment
  • eGFR 50+: No dose adjustment required for eGFR ≥50 mL/min; ensure adequate hydration.
  • eGFR 25–49: Give usual dose every 12 hours (based on creatinine clearance).
  • eGFR 10–24: Give usual dose every 24 hours (based on creatinine clearance).
  • eGFR 0–9: Give half the usual dose every 24 hours (based on creatinine clearance).
Monitoring, pregnancy & toxicity
  • Serum creatinine / eGFR · Baseline and periodically during treatment
  • Hydration status · Throughout treatment; ensure adequate fluid intake
  • Neurological status · Monitor for confusion, hallucinations, or seizures, especially in renal impairment
  • Injection site (IV use) · Monitor for phlebitis during and after infusion
  • Pregnancy: Can be used; the risk of birth defects is not increased.
  • Toxicity: Neurotoxicity: confusion, hallucinations, seizures, coma, especially in renal impairment. · Discontinue or reduce dose; supportive care; consider dialysis in severe cases.
  • Toxicity: Nephrotoxicity: acute kidney injury, crystalluria, flank pain. · Hydration, reduce dose; consider dialysis.
  • Toxicity: IV infusion reactions: phlebitis, pain at injection site. · Ensure adequate dilution, change infusion site, apply local measures.
  • Toxicity: Overdose: nausea, vomiting, diarrhoea, and above neurological effects. · Symptomatic treatment; acyclovir is dialysable; maintain hydration.
Clinical notes
Acyclovir is used for the treatment and prophylaxis of herpes simplex infections (genital, oral, orolabial, and in immunocompromised patients), herpes encephalitis, neonatal herpes, and varicella-zoster virus infections including chickenpox and shingles. It is available in oral, intravenous, and topical formulations. For severe infections (e.g., encephalitis, neonatal herpes) intravenous therapy is required. Common adverse effects include nausea, diarrhoea, and headache; intravenous use can cause nephrotoxicity, so ensure adequate hydration and adjust dose in renal impairment. Topical acyclovir is used for herpes labialis and genital herpes. Early initiation of therapy is important for best outcomes. Resistance can occur, particularly in immunocompromised patients. Always consult local guidelines for dosing and indications.

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BNF; local formulary