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Carbapenems show recognized instability in aqueous solutions; therefore some care must be taken in their handling and preparation and their use in the hospital environment. The stability and degradation products of imipenem were investigated from conditions that simulate its clinical use. For this, a simple stability-indicating method by HPLC-DAD was validated with a focus on the quantitation of drug concentration remaining from infusion solutions (sodium chloride 0.9% and glucose 5%). The degradation products formed were identified by high-resolution mass spectrometry (ESI-Q-TOF-MS/MS), with detection of the [M + H]+ ions at m/z 318 (DP-1), m/z 599 (DP-2) and m/z 658 (DP-3). The most probable elemental compositions were obtained with a high degree of confidence, where the error between the masses observed and calculated was 1.25 ppm for DP-1, -0.33 ppm for DP-2 and 1.82 ppm for DP-3. The DP-1 degradation product resulted from cleavage of the β-lactam ring; DP-2 corresponded to the drug dimer; and DP-3 was generated from the interaction between imipenem and cilastatin. The proposed method provides a safe and reliable alternative for the quantitation of imipenem, and the stability data obtained by ESI-Q-TOF help in understanding the drug behavior under the conditions of clinical use.