Two sizes of acetaminophen injection, 500 and 1,000 mg, have been approved for use in the USA and Europe, while only 1,000 mg has been approved in Japan. In Japan, overdosage of intravenous acetaminophen has been reported in pediatric patients. It is necessary align with international practices and introduce a 500 mg dosage, as in the USA and Europe. Therefore, we conducted a 6-month survey at Kyoto Chubu Medical Center from October 2020, focusing on single dose intravenous acetaminophen. Out of 400 cases, 333 (83.3%) received a single dose of 1,000 mg, 49 cases (12.3%) received 500 mg. Among children (< 15 years), all 17 cases (100%) received a single dose of 500 mg or less; in adults (≥ 15 years), out of 383 cases, 45 cases (11.7%) received 500 mg or less. The most commonly used single dose of intravenous acetaminophen was 1,000 mg, followed by 500 mg. These findings suggest that 1,000 mg is less necessary in children if 500 mg is available, and that, in adults, 1,000 mg may not always be required. In Japan, to avoid overdose in children and optimize dosage in adults, early introduction of 500 mg intravenous acetaminophen is crucial.