Breast cancer is the most common type of cancer among women worldwide. Short-term postsurgical recovery is complicated by many factors, including imbalanced inflammatory and immune response, acute pain associated with functional impairment, and chronic postmastectomy pain (CPMP), developed by about 25-60% of patients. Opioids, most common drugs used for treatment of cancer pain, are immunosuppressive, and therefore, they might directly and/or indirectly influence long-term cancer recurrence. Moreover, they also produce endocrinopathy, which consists primarily of hypothalamic-pituitary-gonadal axis or hypothalamic-pituitary-adrenal axis dysfunction. The interindividual variability in both CPMP and opioid response is believed to be largely underlined by genetic variability in the gene locus for μ-opioid receptor (OPRM1) that modulates opioid pharmacodynamics. For this reason, OPRM1 genotype may play a key role both in short-term postmastectomy outcome and in long-term follow-up, becoming a new biomarker for breast cancer recurrence in patients suffering from chronic postmastectomy pain managed by opioid therapy. Hence OPRM1 might be used in near future to customize the opioid therapy, avoiding not only opioid side effects but also the disease progression. In this review we evaluate the literature state of the art on this topic and possible steps towards obtaining the safest individualized postmastectomy analgesic therapy. Therefore, a personalized pain treatment strategy might be useful to both manage pain and control cancer disease progression.