Strong opioids are useful for managing cancer pain, and common side effects include nausea, vomiting, drowsiness, and constipation. Opioid overdose is known to cause respiratory depression and disturbance of consciousness. We observed a 79-year-old man with stage IVB hepatocellular carcinoma with metastasis to the rib and cancer-related pain being treated with strong opioids who, in spite of receiving a small dose and start volume, experienced strong nausea, photophobia, disturbance of consciousness, and miosis. Oxycodone was stopped to manage the strong nausea, and morphine was stopped to manage the photophobia. Furthermore, fentanyl patch was stopped to manage the disturbance of consciousness and miosis. He did not experience respiratory depression. Thus, photophobia, disturbance of consciousness, and miosis can appear as side effects even at small doses of strong opioids. The results reveal two important clinical issues: (1) photophobia can arise as a side effect of strong opioids, and (2) if photophobia, miosis, disturbance of consciousness arise in opioid-treated patients, they require careful monitoring.