ABSTRACT
Aciclovir-induced neurotoxicity results from the accumulation of aciclovir and its metabolite 9-carboxymethoxymethylguanine (CMMG). It occurs predominantly in older patients with impaired renal function and is characterised by a combination of confusion and psychiatric changes. Seizures, myoclonus and dysarthria may also occur. Critically, peritoneal dialysis has little effect on reversing the toxic effects of aciclovir. We describe a woman in her 70s with renal failure who developed confusion and seizures after receiving aciclovir. She was ultimately diagnosed with aciclovir-induced neurotoxicity, confirmed by an elevated serum CMMG concentration. This condition is likely to be underdiagnosed and the neurologist's primary challenge is differentiating aciclovir-induced neurotoxicity from viral encephalitis.