Acyclovir is well tolerated, but renal adjustments and allergy screening are crucial 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.
Recurrent severe rash, angioedema, or anaphylaxis requires selection of an antiviral from a different class.
Crystalluria and acute kidney injury are dose-related; adjust dosing, optimise hydration, and monitor renal function.
Agents such as tacrolimus increase CNS and renal adverse events; observe for neurological symptoms and adjust therapy promptly.
High-dose IV therapy has precipitated encephalopathy; monitor mental status and consider alternative antivirals if risk is high.