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Purpose: To evaluate the incidence and risk factors of late adverse reactions (ARs) to non-ionic low-osmolar contrast media (LOCM).Methods: The occurrence of late AR was monitored on day 1 and from day 7 to day 28 in all patients who received enhanced computed tomography using LOCM during a 5-week study period in a single tertiary hospital. Patients who experienced late AR were followed up for three years.Results: Among the total 10,540 LOCM exposures, 315 ARs (3.0%) were reported; acute ARs occurred in 108 LOCM exposures (1.0%) and late ARs occurred in 207 LOCM exposures (2.0%) (90.9% within one week, 9.1% developed afterwards by day 20). Previous history of drug allergy (odds ratio [OR] = 4.59; 95% confidence interval [CI] 2.17-9.71) and allergic diseases (OR = 2.54; 95% CI 1.32-4.91) were risk factors of late ARs to LOCM. Although the recurrence rate was lowered with premedication from 8.5% to 1.7% (8/94 vs. 3/178; p = 0.016), LOCM change did not make difference compared to reuse of the culprit LOCM (2/38 vs. 9/234; p = 0.655). In patients with a history of late AR to LOCM, the risk of recurrent reactions decreased with longer time intervals between exposures (OR = 0.86; 95% CI: 0.77-0.97; p = 0.025) and with the use of antihistamine premedication (OR = 0.27; 95% CI: 0.06-0.99; p = 0.049).Conclusions: Late ARs to LOCM occurred mostly within one week. The use of premedication may be helpful in reducing the recurrence of late ARs. [ABSTRACT FROM AUTHOR]