Background: Immunomodulatory peptides are capable of stimulating or suppressing the immune system. Hence, deregulation of them can be considered as an immunotherapy approach. These peptides may have dual behavior in response to different infections. For instance, an antimicrobial peptide may act as an anticancer, tumor marker or even cause cancer progression. Methods: In this study, we used single-cell RNA sequencing analysis to investigate the deregulation of immunomodulatory peptides in malignant versus normal human breast epithelial cells. We validated the obtained results in chromatin accessibility level. Furthermore, we used a drug repositioning approach to change the expression of these peptides based on their role in cancer biology. Results: As a result, ten immunomodulatory peptides were upregulated in breast cancer versus normal. Chromatin was more accessible for these peptides in cancer cell lines versus normal. Among these ten peptides, five of them were tumor drivers (i.e., BST2, GAPDH, S100A8, S100A9 and HMGB1), three of them were anticancer (i.e., H2AFJ, SCGB2A1 and HMGN2), S100A7 had dual behavior in different cancers and ZG16B was a tumor marker. Using the LINCS L1000 database, we proposed a list of drugs that can deregulate the candidate peptides according to their role in the progression of malignancy. Conclusions: In conclusion, immunomodulatory peptides can be considered as drug targets based on their role in cancer biology.