Inappropriate Valacyclovir Dosing by Renal Function in Japan

Background: Valacyclovir requires dose adjustment according to renal function to prevent serious adverse events. However, real-world data on renal function-based dosing practices are limited. Objective: To evaluate the frequency of valacyclovir overdosing according to renal function and to identify the characteristics associated with overdosing in Japan. Methods: We conducted a retrospective cohort study using a nationwide database of administrative claims and laboratory results from acute care hospitals in Japan. We included adults aged ≥18 years who received at least 1 valacyclovir prescription between October 1, 2022, and September 30, 2024, and who had a serum creatinine measurement within the preceding year. The primary outcome was overdosing, defined as a prescribed dose that exceeded the maximum recommended dose for the patient’s renal function level per the Japanese package insert. Renal function was estimated using creatinine clearance calculated with the Cockcroft-Gault equation. Results: A total of 6764 patients with 7901 valacyclovir prescriptions were included; 1649 prescriptions (20.9%) were for patients with impaired renal function. Overdosing was rare among patients with normal renal function. Among patients with impaired renal function, overdosing occurred in 25.1% of prescriptions for varicella or herpes zoster and in 6.4% for other indications. Overdosing was more frequent among women and most common for prescriptions dispensed at community pharmacies. Conclusions: In this nationwide study, valacyclovir overdosing was common among patients with impaired renal function, particularly for high-dose indications, and occurred more frequently among women and for prescriptions dispensed at community pharmacies. Strengthening renal function-based prescription review may help reduce preventable overdosing.