OBJECTIVE To analyze the correlation of the peak blood concentration （cmax） of compound sulfamethoxazole （TMP/SMZ） and its metabolite N-acetyl sulfamethoxazole （NSMZ） with clinical efficacy and adverse reactions in critically ill patients. METHODS The data of critically ill patients treated with TMP/SMZ in various ICU of Hainan General Hospital from December 2023 to January 2025 were retrospectively collected. The patients were divided into success group and failure group based on the treatment outcome. Simple linear regression and Spearman correlation analysis were used to analyze the correlation of TMP cmax， SMZ cmax， and NSMZ cmax with clinical efficacy and adverse reactions. The receiver operating characteristic curve （ROC） was used to determine the cutoff values of cmax for predicting the occurrence of adverse reactions. RESULTS Among critically ill patients with an acute physiology and chronic health evaluation Ⅱ （APACHE-Ⅱ） ≥15 points 24 h of check-in at ICU， SMZ cmax of success group was significantly higher than failure group （P＜0.05）. The daily total dose of TMP/SMZ was positively correlated with TMP cmax and SMZ cmax（ P＜0.05）. TMP cmax was significantly correlated with hepatotoxicity and nephrotoxicity， SMZ cmax with hepatotoxicity， and NSMZ cmax with nephrotoxicity （P＜0.05）. The cutoff values of TMP cmax for predicting nephrotoxicity and hepatotoxicity were 7.25 μg/mL and 6.63 μg/mL， respectively. The cutoff value of SMZ cmax for predicting hepatotoxicity was 138.00 μg/mL， and that of NSMZ cmax for predicting nephrotoxicity was 60.76 μg/mL. CONCLUSIONS Among critically ill patients with an APACHE-Ⅱ ≥15 points 24 h of check-in at ICU， SMZ cmax is associated with treatment success. Hepatotoxicity risk significantly increases when TMP cmax ≥6.63 μg/mL or SMZ cmax ≥138.00 μg/mL； nephrotoxicity risk significantly increases when TMP cmax ≥7.25 μg/mL or NSMZ cmax ≥60.76 μg/mL.