This study examines opioid-related deaths in England and Wales from 2011 to 2022 using data from the Office for National Statistics (ONS) and the National Programme on Substance Use Mortality (NPSUM). Findings indicate that ONS data significantly underestimates opioid-related deaths due to ambiguous cause-of-death reporting, particularly amid rising polydrug use. In 2011, heroin- and morphine-related deaths were underestimated by 29 percent, increasing to 36 percent by 2022. Similar underestimations were observed for other opioids, including methadone and fentanyl. These discrepancies have important implications for resource allocation, policymaking, and public health strategies to address opioid-related mortality.