BACKGROUND: Globally, opioid drug use is an important cause of premature mortality. In many countries, opioid using populations are ageing. The current study investigates mortality in a large cohort of opioid users; with a focus on testing whether excess mortality changes with age. METHODS: 198,247 opioid users in England were identified from drug treatment and criminal justice sources (April, 2005 to March, 2009) and linked to mortality records. Mortality rates and standardised mortality ratios (SMRs) were calculated by age-group and gender. RESULTS: There were 3974 deaths from all causes (SMR 5.7, 95% Confidence Interval: 5.5 to 5.9). Drug-related poisonings (1715) accounted for 43% of deaths. Relative to gender-and-age-appropriate expectation, mortality was elevated for a range of major causes including: infectious, respiratory, circulatory, liver disease, suicide, and homicide. Drug-related poisoning mortality risk continued to increase beyond 45 years and there were age-related increases in SMRs for specific causes of death (infectious, cancer, liver cirrhosis, and homicide). A gender by age-group interaction revealed that whilst men have a greater drug-related poisoning mortality risk than women at younger ages, the difference narrows with increasing age. CONCLUSION: Opioid users' excess mortality persists into old age and for some causes is exacerbated. This study highlights the importance of managing the complex health needs of older opioid users.