Duloxetine hydrochloride, with CAS number 116539-59-4 for the API and 136434-34-9 for the free base, is a selective serotonin and norepinephrine reuptake inhibitor (SNRI). Its chemical name is (+)-(S)-N-Methyl-3-(naphthalen-1-yloxy)-3-(thiophen-2-yl)propan-1-amine hydrochloride. The compound specified is identified as a related substance or impurity of duloxetine, specifically the (R)-enantiomer of the free base amine, often referred to as the (R)-desmethyl impurity or duloxetine impurity C. As an impurity standard, this compound is critical in the pharmaceutical quality control process for duloxetine. It is used as a reference standard in analytical methods, such as high-performance liquid chromatography (HPLC), to identify, quantify, and monitor this specific enantiomeric impurity during the manufacturing of the active pharmaceutical ingredient (API) and in the final drug product. Controlling such impurities is essential to ensure the safety, efficacy, and regulatory compliance of the antidepressant medication. Duloxetine itself is a widely prescribed API for the treatment of major depressive disorder (MDD), generalized anxiety disorder (GAD), neuropathic pain, fibromyalgia, and chronic musculoskeletal pain. The strict control of its enantiomeric purity is paramount due to the potential for different pharmacological activities or toxicities associated with the undesired stereoisomer.