MicroRNAs (miRs) regulate gene expression at the post-transcriptional level and are found to be present in monocytes. This study aimed to investigate miR-221-5p, miR-21-5p, and miR-155-5p, their expression in monocytes, and their role in coronary arterial disease (CAD). The study population comprised 110 subjects, and RT-qPCR was used to examine the miR-221-5p, miR-21-5p, and miR-155-5p expressions in monocytes. Results: the miR-21-5p (p= 0.001) and miR-221-5p (p< 0.001) expression levels were significantly higher in the CAD group, and the miR-155-5p (p= 0.021) expression levels were significantly lower in the CAD group; only miR-21-5p and miR-221-5p upregulation was found to be associated with an increased CAD risk. The results show significant increases in miR-21-5p in the unmedicated CAD group with the metformin patients vs. the healthy control group (p= 0.001) and vs. the medicated CAD group with metformin (p= 0.022). The same was true for miR-221-5p in the CAD patients unmedicated with metformin vs. the healthy control group (p< 0.001). Our results from Mexican CAD patients show that the overexpression in monocytes of miR-21-5p and miR-221-5p increases the risk of the development of CAD. In addition, in the CAD group, the metformin downregulated the expression of miR-21-5p and miR-221-5p. Also, the expression of endothelial nitric oxide synthase (NOS3) decreased significantly in our patients with CAD, regardless of whether they were medicated. Therefore, our findings allow for the proposal of new therapeutic strategies for the diagnosis and prognosis of CAD and the evaluation of treatment efficacy.