Acyclovir Contraindications & InteractionsWhen Not to Use & Important Drug Interactions
Acyclovir is well tolerated, but renal adjustments and allergy screening are important before starting therapy in children.
Absolute contraindication is hypersensitivity to acyclovir or valacyclovir. Relative precautions include renal impairment, dehydration, concomitant nephrotoxic drugs, and underlying neurologic disease that could predispose to neurotoxicity.
Contraindications
Absolute Contraindications (Never Use)
True hypersensitivity to acyclovir/valacyclovir
Risk of rapid-onset urticaria, angioedema, or anaphylaxis; rechallenge not recommended outside desensitization programs.
Relative Contraindications (Use With Caution)
Renal impairment or dehydration
Crystalluria and acute kidney injury can develop rapidly when serum levels accumulate.
If used, monitor: Check creatinine, adjust dose/interval, maintain ≥1 mL/kg/hr urine output, hold concurrently nephrotoxic agents when feasible.
Neurologic disorders
High serum levels can precipitate encephalopathy, hallucinations, or seizures.
If used, monitor: Lower doses, monitor mental status daily, and pause therapy for unexplained neurologic changes.
Drug Interactions
Probenecid
moderate interactionSpecific drugs: Probenecid
Effect: Increased risk of nephrotoxicity and neurotoxicity.
How it happens: Inhibits renal tubular secretion of acyclovir, raising serum concentrations.
Management: Avoid combination when possible; otherwise reduce acyclovir dose and monitor creatinine q48h.
Lithium
moderate interactionSpecific drugs: Lithium
Effect: Lithium toxicity (tremor, ataxia, vomiting).
How it happens: Competition for renal excretion increases lithium levels.
Management: Check lithium levels after 2–3 days of concomitant therapy; adjust lithium dose or choose alternative antiviral.
Calcineurin inhibitors (tacrolimus, cyclosporine)
moderate interactionSpecific drugs: Calcineurin inhibitors (tacrolimus, cyclosporine)
Effect: Rapid rise in creatinine and electrolyte disturbances.
How it happens: Additive nephrotoxicity with reduced renal perfusion.
Management: Ensure IV hydration, stagger dosing, monitor kidney function twice weekly.
NSAIDs at high doses
moderate interactionSpecific drugs: NSAIDs at high doses
Effect: Acute kidney injury in dehydrated adolescents.
How it happens: Decreased renal blood flow compounds crystalluria risk.
Management: Limit NSAID use; use acetaminophen for pain/fever when possible.
Monitoring
Baseline Guidance
- Serum creatinine/eGFR
- Hydration status (intake/output, mucous membranes)
- Allergy history (penicillin rash does not cross-react)
During Therapy
- Renal function for courses >5 days or any IV therapy
- Neurologic assessment (sleep patterns, behavior, hallucinations)
- Hydration status and urine output
Red Flags
- Anaphylaxis or angioedema
- Acute kidney injury (reduced urine output, flank pain)
- Neurotoxicity (confusion, tremors, hallucinations)
Parent Communication Guide
Key Safety Messages
- Encourage your child to drink plenty of fluids while taking acyclovir.
- Report confusion, dizziness, or decreased urination immediately.
- Store the medication out of reach; dosing more often than prescribed does not speed recovery.