BackgroundAzithromycin has favorable effects on critical respiratory diseases owing to its antimicrobial and anti-inflammatory properties. During the early stages of the coronavirus disease 2019 (COVID-19) pandemic, azithromycin was frequently administered before specific treatments were developed. However, the efficacy of this treatment has not been verified. We retrospectively investigated the effects of its intravenous (IV) administration in patients with severe/critical COVID-19 using the National Administrative Database of Japan during the first wave (February–April 2020). MethodsPatients were categorized based on whether they received IV azithromycin within three days of hospitalization. An overlap weighting method with estimated propensity scores was used to reduce bias. ResultsAmong the 830 patients with severe/critical COVID-19, 148 (17.8%) received azithromycin, and 682 (82.2%) did not. After adjustment, the use of azithromycin was associated with a shorter duration of intensive care unit (ICU) management (-3.48 days, 95% confidence interval [CI]: -4.59 to -2.38). However, other endpoints, including mortality rate, duration of mechanical ventilation, and duration of hospital stay, did not suggest any associations. Furthermore, of the 115 ICU patients, 27 (23.5%) were treated with IV azithromycin and 88 (76.5%) were not. After adjustment, azithromycin was associated with favorable outcomes, including reduced in-hospital mortality (odds ratio [OR], 0.45, 95% CI: 0.22 to 0.92), 30-day mortality (OR, 0.46, 95% CI: 0.22 to 0.94), and a shorter duration of ICU management (-2.94 days, 95% CI: -5.15 to -0.73). ConclusionWe verified that IV azithromycin was associated with favorable impact in patients with COVID-19 requiring ICU management.