A 71-year-old woman with normal renal function one month prior to presentation was diagnosed with shingles on her right thigh and received valacyclovir (VACV 3,000 mg/day). She was subsequently admitted to the hospital for evaluation of renal dysfunction and impaired consciousness. Acyclovir (ACV) neurotoxicity was suspected, and VACV was discontinued with initiation of continuous renal replacement therapy. Her symptoms resolved. Blood tests revealed a blood ACV level of 57.5 μg/mL. The patient was diagnosed with ACV neurotoxicity based on her clinical course. Clinicians should be mindful that renal dysfunction and ACV neurotoxicity may occur even in patients with normal renal function before treatment. Among all reported cases of ACV neurotoxicity in patients with normal renal function before VACV administration, 9 of 10 patients were aged > 65 years. Thus, these studies support our findings that elderly patients warrant special attention. ACV neurotoxicity symptoms are nonspecific but may improve with early diagnosis and drug discontinuation. The incidence of shingles is increasing in recent years, and the number of VACV prescriptions in primary care is expected to increase. Primary care physicians should be familiar with ACV neurotoxicity to ensure the prompt diagnosis of this condition.