- OBJECTIVES: To examine the risk of bowel obstruction in older adults during treatment with extended-release nifedipine compared with patients treated with amlodipine. DESIGN: Retrospective cohort study using multiple linked healthcare databases. SETTING: Ontario, Canada from 1 April 1997 to 31 December 2010. PARTICIPANTS: We identified patients aged 66 years and older who started treatment with Adalat XL, an extended-release nifedipine product employing a tablet delivery system associated with mechanical bowel obstruction. For comparison, we studied patients receiving amlodipine, a long-acting calcium channel blocker that does not utilise the same delivery system and has not been implicated as a cause of bowel obstruction. Propensity score matching was used to ensure similarity of patients receiving the two drugs. PRIMARY OUTCOME MEASURE: HR for the association between extended-release nifedipine relative to amlodipine and hospitalisation for bowel obstruction during therapy. RESULTS: Over the 13-year study period, we identified 103 657 patients treated with extended-release nifedipine and 204 733 patients treated with amlodipine. In these two groups, 591 (0.6%) and 1185 (0.6%) of patients were hospitalised for bowel obstruction, respectively. We found no difference in the risk of bowel obstruction among patients treated with extended-release nifedipine compared with amlodipine (HR 1.09, 95% CI 0.96 to 1.24). CONCLUSIONS: Bowel obstruction during treatment with extended-release nidefipine is rare, and the risk is not appreciably greater than that during treatment with amlodipine.