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Research published by The BMJ adds to evidence that rosiglitazone, used to treat type 2 diabetes, is associated with increased risk of heart problems, especially heart failure. Rosiglitazone, a thiazolidinedione, helps control blood sugar levels in patients with type 2 diabetes, but can also increase the risk of serious heart problems, leading to suspension of its use in Europe and to earlier restrictions in the US. However, conflicting findings have been reported since 2007 of meta‐analyses of studies investigating whether rosiglitazone increases the risk of heart attack. The relevant studies did not have access to individual patient level data from clinical trials, instead relying on summary level data (ie, results reported in publications and clinical trial registries), which are not as reliable for assessing the safety profile of drugs. US researchers have now analysed the results of more than 130 trials, involving more than 48 000 adult patients, that compared rosiglitazone with any control for at least 24 weeks. Individual patient level data were available for 33 of the trials (21 156 patients), analysis of which indicated that rosiglitazone was associated with a 33% higher risk of a composite cardiovascular event (heart attack, heart failure, cardiovascular‐ and non‐cardiovascular‐related death) compared with controls (274 events among 11 837 patients taking rosiglitazone and 219 events among 9319 control patients). In particular, analysis of the 33 trials with individual patient level data yielded higher estimates of the risk of heart attacks than did those of trials reporting patient or summary level data. These findings highlight the potential for differing results to be derived from different types of data, and underline the need for greater clinical trial transparency and data sharing to accurately assess the safety of drugs, say the researchers.