Cis-Montelukast is a stereoisomer of Montelukast sodium, a well-known leukotriene receptor antagonist. Its chemical name is 1-[[[(1R)-1-[3-[(1Z)-2-(7-Chloro-2-quinolinyl)ethenyl]phenyl]-3-[2-(1-hydroxy-1-methylethyl)phenyl]propyl]thio]methyl]cyclopropaneacetic acid. This compound is structurally characterized by a quinoline moiety, a thioether linkage, and a cyclopropaneacetic acid group. The primary significance of cis-Montelukast lies in the pharmaceutical industry as a process-related impurity or a degradation product of the active pharmaceutical ingredient (API) Montelukast sodium. Montelukast sodium is a blockbuster drug used for the prophylaxis and chronic treatment of asthma and allergic rhinitis. Therefore, cis-Montelukast is critical for analytical method development, quality control, and regulatory compliance, serving as a reference standard to monitor and control impurity levels during API manufacturing and in final drug products. As a specified impurity, its identification, quantification, and control are mandated by pharmacopoeial standards (e.g., USP, EP) to ensure the safety, efficacy, and purity of the final medicinal product. It is not used therapeutically itself but is essential for maintaining the high-quality standards of the respiratory drug Montelukast.