Background <p>Smoking impairs spermatogenesis by disrupting gene and miRNA expression profiles. This study aims to explore the protective effects of quercetin against nicotine-induced testicular damage, with a specific focus on its regulatory role on sperm miR-151a-5p and testicular Cep72 gene expression, as potential molecular mechanisms involved in male reproductive dysfunction.</p> <p>Male BALB/c mice (<i>N</i> = 32) were randomly divided into four groups of: control, nicotine, quercetin, and quercetin + nicotine combined groups. Treatments lasted 35 days. Spermatogenesis was evaluated through histopathological studies of the testicles. TAC and SOD levels were evaluated as markers of antioxidant activity using colorimetric methods in testicular homogenates. Sex hormones were measured using the ELISA method. Relative expression of Cep72 and miR-151a-5p genes in testicular tissue and epididymal sperm was assessed by real-time PCR. Androgen receptor (AR) and estrogen receptor alpha (ERα) expression in testicular tissue were evaluated by immunohistochemical methods.</p> Results <p>The combined treatment of quercetin and nicotine enhanced sperm quality, caused significant changes in LH and estradiol hormone levels, increased anti-oxidants in testicular homogenates, and increased AR expression in Sertoli cells without affecting ERα, compared to the nicotine group. In addition, the combined therapy improved spermatogenesis by increasing the number of germ cells and Leydig cells. Furthermore, combined therapy increased testicular Cep72 gene expression and reduced sperm miR-151a-5p.</p> Conclusions <p>This study demonstrates that quercetin may protect against nicotine-induced testicular damage by maintaining hormonal balance, enhancing AR expression, improving antioxidant enzyme activity, and normalizing the miR-151a-5p/Cep72 regulatory axis, ultimately supporting sperm quality and spermatogenesis. These results establish a foundation for future mechanistic and translational research.</p>