Opioid-induced constipation (OIC) is one of the most frequent and distressing adverse effects of opioid therapy. Unlike other opioid-related side effects that improve with tolerance, OIC persists and significantly impairs quality of life, treatment adherence, and health economics. In Japan, the incidence of OIC has been reported at 40–65%, highlighting its clinical burden. Preventive strategies, including prophylactic laxatives and peripherally acting μ-opioid receptor antagonists such as naldemedine, are recommended to reduce its impact. Tapentadol, with its dual mechanism of action, may also lower the risk of constipation. In contrast, narcotic bowel syndrome (NBS) is a less recognized but clinically important manifestation of opioid-induced bowel dysfunction, characterized by chronic or recurrent abdominal pain that paradoxically worsens with continued or escalated opioid use. NBS often mimics functional or obstructive gastrointestinal disorders, leading to diagnostic delay and inappropriate dose escalation. Early recognition, structured opioid tapering, symptomatic pharmacologic therapy, and psychological support are essential for management. As opioid prescriptions increase beyond cancer-related pain to chronic nonmalignant conditions, greater clinical awareness of both OIC and NBS is crucial. Optimized diagnosis, prevention, and treatment strategies are indispensable to improve patient outcomes and ensure safe opioid use.