How Acyclovir Works in the Body
Acyclovir is an antiviral drug that mainly targets herpes viruses by inhibiting viral DNA replication. It concentrates in tissues infected by these viruses, such as skin lesions, mucous membranes, and nerves, rather than broadly affecting healthy organs.[1]
Primary Organs of Metabolism and Clearance
The kidneys handle most acyclovir processing—about 70-80% of the dose is excreted unchanged in urine via glomerular filtration and tubular secretion. This makes renal function critical for safe use.[1][2]
What Happens in Patients with Kidney Issues
Dose adjustments are required for impaired kidney function to prevent accumulation, which can lead to neurotoxicity like confusion or hallucinations. Liver metabolism is minor (less than 15%), so it rarely causes issues.[1][2]
Common Side Effects by Organ System
- Nervous system: Headache, dizziness, or tremors (1-10% of patients).
- Gastrointestinal: Nausea, vomiting, diarrhea (less common).
- Skin: Rash or itching at injection sites (intravenous use).
Rarely affects heart, lungs, or blood-forming organs unless overdosed.[2]
Risks from High Doses or IV Administration
Excess acyclovir can cause acute kidney injury (crystal nephropathy) in 5-12.5% of high-risk IV cases, reversible with hydration. Encephalopathy occurs in severe overdose, mainly impacting the brain.[1][2]
Who Should Avoid It and Alternatives
Avoid in severe kidney failure without adjustment. Alternatives like valacyclovir (prodrug of acyclovir) have similar profiles but better oral absorption.[2]
[1]: FDA Label for Acyclovir
[2]: UpToDate: Acyclovir Pharmacology