Dr. Jeffrey Johnson of the School of Public Health at the University of Alberta and colleagues conducted a systematic review and meta-analysis of randomized-controlled trials and observational studies involving more than 2.6 million patients.
People with type 2 diabetes are at risk of several types of cancer, including a 40 percent increased risk of bladder cancer, compared with people without diabetes, said Johnson, the study co-author.
Previous studies showed a higher incidence of bladder cancer in people taking pioglitazone, a type of thiazolidinedione -- a class of medications used in the treatment of type 2 diabetes introduced in the late 1990s.
"We observed an increased risk of bladder cancer associated with the use of thiazolidinediones," Johnson said in a statement. "In particular, use of pioglitazone was associated with an increased risk of bladder cancer based on a pooled estimate from three cohort studies involving more than 1.7 million individuals."
However, the study, published in the Canadian Medical Association Journal, also looked for an association with the medication rosiglitazone, another type of thiazolidinedione, but did not see an effect.
"Although the absolute risk of bladder cancer associated with pioglitazone was small, other evidence-based treatments for type 2 diabetes may be equally effective and do not carry a risk of cancer," the research team said.
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