Background Gastrointestinal side effects are commonly associated with opioid treatment for pain. Aim To understand gastrointestinal side effects associated with opioid treatment. Methods This study was a population-based survey of adults in the US who use opioids to manage pain unrelated to cancer. Participants were recruited from an existing Web-enabled panel and a supplemental panel of individuals who previously indicated an interest in participating in Web-based surveys. Results Overall, 2055 individuals participated in the main phase of the survey. Fifty-seven per cent of participants reported having had constipation that they associated with opioid treatment, and 49% reported constipation in the previous 4 weeks. Thirty-six per cent of participants reported new or worsening constipation in the previous 4 weeks. Thirty-three per cent of participants reported constipation as their most bothersome symptom associated with opioid treatment, 13% reported nausea, 11% abdominal pain and 10% gas. Seventy-three per cent of the participants who reported any GI symptoms did not change the dosage level or frequency of use of opioids because of adverse events, which may be explained by the fact that 72% of participants used over-the-counter laxatives and 12% a prescription laxative. Conclusion Constipation is a frequent and significant event occurring with opioid use.