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Background: Cefaclor, a second-generation oral cephalosporin, is widely prescribed to treat infectious diseases. Immediate hypersensitivity (HS) reactions to cefaclor have continuously been reported and are expected to increase with its greater use. This study aimed to investigate the clinical characteristics and risk factors of immediate HS to cefaclor over the most recent 5 years. Methods: This retrospective study investigated 521 adverse drug reactions (ADRs) to cefaclor at pharmacovigilance centers at two tertiary hospitals from January 2014 to December 2018. In total, 459 patients with immediate HS to cefaclor were reviewed. Results: A total of 459 cases of cefaclor immediate HS were included among 521 cefaclor ADRs, and anaphylaxis was recorded in 61.2%. Female sex (odds ratio 2.917, 95% confidence interval 2.397–3.550, P < 0.001), age under 65 years (4.225, 3.017–5.916, P < 0.001), hypertension (2.520, 1.875–3.388, P < 0.001), liver diseases (2.189, 1.208–3.967, P = 0.010), asthma (8.075, 5.301–12.302, P < 0.001), and concomitant use of nonsteroidal anti-inflammatory drugs (1.888, 1.554–2.294, P < 0.001) were significantly associated with cefaclor immediate HS. Conclusions: Cefaclor was found to elicit high proportions of immediate HS and anaphylaxis. Physicians ought to be cautious with prescribing cefaclor to females, individuals with hypertension, liver diseases, or asthma, and patients taking nonsteroidal anti-inflammatory drugs. Trial registration: This study was retrospectively registered. [ABSTRACT FROM AUTHOR]