Common Side Effects of Acyclovir
Acyclovir, an antiviral used for herpes infections, typically causes mild reactions like nausea, vomiting, diarrhea, headache, and dizziness. Skin reactions such as rash or itching occur in about 1-5% of patients.[1]
What Counts as Unusual Reactions?
Unusual reactions are less frequent or severe events reported in clinical data and post-marketing surveillance. These include:
- Neurological effects: Confusion, hallucinations, agitation, tremors, or seizures, especially in older adults or those with kidney issues (incidence <1%).[1][2]
- Kidney problems: Elevated creatinine, acute kidney injury from crystal formation in urine, more common with high IV doses or dehydration.[2]
- Blood disorders: Rarely, thrombocytopenia, anemia, or leukopenia.[1]
- Allergic responses: Angioedema, urticaria, or anaphylaxis in hypersensitive individuals.[3]
Why Do These Happen and Who’s at Risk?
Acyclovir is excreted by the kidneys, so impaired renal function increases toxicity risk—dose adjustments are required for creatinine clearance below 50 mL/min. IV administration heightens neurotoxicity odds compared to oral forms. Elderly patients, those on nephrotoxic drugs (e.g., NSAIDs), or dehydrated individuals face higher risks.[2][4]
How Rare Are They Compared to Normal Reactions?
Severe reactions like encephalopathy occur in 1-2% of hospitalized patients on IV acyclovir, versus 10-20% for GI upset. Most resolve after stopping the drug and supportive care.[4]
What Should Patients Watch For?
Seek immediate care for symptoms like seizures, severe confusion, reduced urine output, or swelling. Report persistent unusual symptoms to a doctor—most are reversible if caught early.[1][3]
[1]: FDA Label for Zovirax (Acyclovir)
[2]: MedlinePlus - Acyclovir
[3]: Drugs.com - Acyclovir Side Effects
[4]: UpToDate - Acyclovir Overview