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Zovirax (Acyclovir) for Cold Sores: Effectiveness, Dosing, and Side Effects

Acyclovir (brand Zovirax) is an antiviral medication used to treat herpes simplex virus (HSV-1/HSV-2) infections, including cold sores (herpes labialis). It reduces duration, severity, and viral shedding.

Mechanism of Action

Acyclovir is a guanosine analogue. In HSV-infected cells, viral thymidine kinase phosphorylates it to acyclovir monophosphate; cellular kinases convert it to the active acyclovir triphosphate. This competitively inhibits viral DNA polymerase and incorporates into viral DNA, causing chain termination. Selectivity: Preferentially activated in virus-infected cells → low host toxicity.

Formulations & Dosing for Cold Sores

Pediatric dosing adjusted by weight/age. Renal impairment requires dose reduction.

Side Effects

  • Topical: Mild burning, stinging, dryness, flaking at application site.
  • Oral: Nausea, vomiting, diarrhea, headache, dizziness. Rare: nephrotoxicity (ensure hydration), neurotoxicity (confusion, hallucinations) in renal impairment/elderly.

Special Populations

  • Pregnancy: Category B (no human risk shown). Topical preferred; oral if benefits outweigh risks.
  • Breastfeeding: Low milk levels; generally compatible.
  • Drug Interactions: Probenecid ↑ acyclovir levels; nephrotoxic drugs (aminoglycosides, amphotericin) ↑ renal risk.

Key Points

  • Start treatment at first sign (tingle, itch, redness) for maximal benefit.
  • Does not cure HSV; virus remains latent in trigeminal ganglion.
  • Suppressive therapy reduces recurrence frequency by 70–80%.
  • Valacyclovir (prodrug) and famciclovir are alternatives with better bioavailability/less frequent dosing.

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FormulationTypical Dosing (Adults)Duration
Cream 5% / Ointment 5%Apply to lesion 5× daily (every 3–4 hrs while awake)4–10 days (start at prodrome for best effect)
Oral tablets 200 mg400 mg 5× daily OR 800 mg 3× daily5–10 days (episodic); 400 mg 2× daily (suppressive)
Buccal tablet 50 mg (Sitavig)Single dose applied to upper gumSingle application