An 83-year-old Japanese man had a 29-year history of well-controlled diabetes 
mellitus. His HbA1c level was approximately 6.0%, with no microalbuminuria and a 
serum creatinine level seven days before admission of 0.8 mg/dl (eGFR: 69.67 
ml/min/1.73 m(2)). Five days before admission, he visited an ophthalmologist 
with inflammation of the right palpebra and conjunctiva and began taking 
valacyclovir at a dose of 3,000 mg for the treatment of herpes zoster. Two days 
before admission, he was prescribed loxoprofen at a dose of 180 mg for a 
headache. One day prior to admission, he developed dysarthria, wandering and 
loss of appetite. He was subsequently admitted to our hospital with progressive 
deterioration of consciousness (Japan Coma Scale: II-20). On admission, he 
exhibited renal dysfunction, with a serum creatinine level of 5.11 mg/dl (eGFR: 
9.16 ml/min/1.73 m(2)). Based on his diverse symptoms and current treatment with 
valacyclovir, the patient was diagnosed with acyclovir-induced neurotoxicity and 
his symptoms rapidly improved after hemodialysis. The serum acyclovir level on 
admission was found to be 9.25 μg/ml. Although acyclovir-induced neurotoxicity 
is commonly seen in elderly patients with renal dysfunction, there are also 
reports of this condition in patients with a normal renal function. Valacyclovir 
is frequently prescribed to the elderly to treat diseases such as herpes zoster. 
As valacyclovir induces renal dysfunction, which raises the serum acyclovir 
level to the toxic range, special attention must be paid when administering this 
drug in elderly subjects.