This report describes the prevalence of opioid use in the US adult population, overall and in subgroups, the characteristics of opioid use, and concomitant medication use among opioid users. Data were obtained from the Slone Survey, a population-based random-digit dialing survey. One household member was randomly selected to answer a series of questions regarding all medications taken during the previous week. There were 19,150 subjects aged > or = 18 interviewed from 1998 to 2006. Opioids were used 'regularly' ( > or = 5 days per week for > or = 4 weeks) by 2.0%; an additional 2.9% used opioids less frequently. Regular opioid use increased with age, decreased with education level, and was more common in females and in non-Hispanic whites. The prevalence of regular opioid use increased over time and was highest in the South Central region. Nearly one-fifth of regular users had been taking opioids for > or = 5 years. Concomitant use of > or = 10 non-opioid medications was reported by 21% of regular opioid users compared to 4.5% of subjects who did not use opioids. Regular opioid users were more likely to use stool softeners/laxatives (9% vs. 2%), proton pump inhibitors (25% vs. 8%), and antidepressants (35% vs. 10%). From this nationally-representative telephone survey, we estimate that over 4.3 million US adults are taking opioids regularly in any given week. Information on the prevalence and characteristics of use is important as opioids are one of the most widely prescribed classes of drugs in the US.