Antiviral – nucleoside analogue
ATC code: J05AB01 (Aciclovir)
A guanosine analogue. In virus-infected cells, viral thymidine kinase (TK) phosphorylates aciclovir to the monophosphate; cellular enzymes then convert it to active triphosphate. Aciclovir triphosphate competitively inhibits viral DNA polymerase and incorporates into growing DNA, terminating synthesis. Selectivity comes from absence of viral TK in uninfected cells. Effective against HSV-1, HSV-2, and VZV; less so against EBV and CMV. Half-life 2.5–3 hours.
First line
First-line for HSV-1 and HSV-2 infections of various locations: primary and recurrent genital herpes, herpes encephalitis (IV), neonatal herpes. In primary genital herpes 400 mg three times daily for 7–10 days; recurrences – 800 mg three times daily for 2 days or 400 mg five times daily for 5 days. Suppressive therapy 400 mg twice daily for more than 6 recurrences per year reduces them by 70–80%.
First line
First-line for herpes zoster per 2024 and 2007. Dose 800 mg five times daily for 7 days. Starting within 72 hours of rash onset reduces pain intensity, skin lesion duration, and postherpetic neuralgia risk. Valacyclovir and famciclovir have more convenient three-times-daily dosing with equivalent efficacy and are often preferred.
First line
Used in recurrent herpes labialis. Topical 5% cream is applied 5 times daily for 4 days, starting in prodrome. The Cochrane 2007 meta-analysis showed topical aciclovir reduces episode duration by 0.5–1 day with minimal toxicity. Oral aciclovir 400 mg three times daily for 5 days is more effective in severe recurrences. In patients with more than 6 episodes per year, suppressive therapy 400 mg twice daily long-term is used.
The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.
Not recommended
Aciclovir is an antiviral drug active against herpesviruses (HSV-1, HSV-2, VZV). It is prescribed for infections caused by these viruses – genital and orolabial herpes, chickenpox, and shingles (AEMPS Ficha Técnica). For common colds or flu, aciclovir does not work – respiratory virus pathogens (rhinoviruses, adenoviruses, coronaviruses, influenza) are not susceptible to this drug. Taking aciclovir at the first signs of a cold is not justified. The drug can cause nausea, headache, and acute kidney injury with intravenous administration. If aciclovir was prescribed for a cold, consider seeking a second opinion.
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FDA category B. Large pregnancy data set without described teratogenic effects. Used when indicated (genital herpes pre-delivery to prevent neonatal transmission, herpes zoster).
Compatible with breastfeeding. Moderate milk concentrations but low infant GI absorption.
Aciclovir is evaluated for the following indications with varying evidence strength: Herpes simplex (evidence tier A), Herpes zoster (evidence tier A), Herpes labialis (evidence tier B). See the full indication matrix with dosing and citations above on this page.
Common side effects of Aciclovir (≥ 1 in 100): Nausea, vomiting, Diarrhea, Headache, Fatigue, Skin rash. See the Safety section for uncommon and serious reactions.
FDA category B. FDA category B. Large pregnancy data set without described teratogenic effects. Used when indicated (genital herpes pre-delivery to prevent neonatal transmission, herpes zoster).
Compatible with breastfeeding. Moderate milk concentrations but low infant GI absorption.
Aciclovir is contraindicated in: Hypersensitivity to aciclovir or valacyclovir; Severe renal impairment without dose adjustment. Full list in the Safety section.
Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.