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Researchers from the Perelman School of Medicine at the University of Pennsylvania found that patients taking thiazolidinedione (TZDs) increases patients' risk of bladder cancer. 

The researchers found that patients taking TZDs are two to three times more likely to develop bladder cancer than those who took a sulfonylurea drug, another common class of medications for diabetes.

TZDs account for up to 20 percent of the drugs prescribed to diabetics in the United States.

The study is published online in the Journal of the National Cancer Institute. 

The authors said the findings are especially important since diabetic patients are known to already be at a slightly increased risk of this type of cancer as compared to the generation population, in which about 30 in 100,000 people develop bladder cancer. Among diabetes patients overall, the incidence of this cancer is typically about 40 out of 100,000.

Diabetes is one the most common chronic diseases worldwide, affecting 285 million people.

Researchers analyzed 60,000 Type 2 diabetes patients.

They found that patients treated with the TZD drugs pioglitazone (Actos) or rosiglitzaone (Avandia) for five or more years had a two-to-three-fold increase in risk of developing bladder cancer when compared to those who took sulfonylurea drugs. A

Among patients taking TZDs for that length of time, the team's analysis indicates that 170 patients per 100,000 would be expected to develop the disease. About 60 in 100,000 of those who take sulfonylurea drugs - such as glipizide (Glucotrol) -- would be expected to develop bladder cancer.

In the United States, most diabetic patients no longer take Avandia since it was linked to severe cardiovascular problems. The FDA had warned that it may be associated with a risk of bladder cancer, and France and Germany have removed the drug from their markets. 

The new findings add to mounting evidence against the entire class of TZDs, as one of the first studies examining this type of risk among people taking both types of TZDs and among those taking sulfonylurea drugs.

Actos is the ninth most commonly prescribed drug in the nation, accounting for some 15 million prescriptions each year. The drug is a common choice when Type 2 diabetes patients' illnesses can no longer be controlled with the first-line diabetes drug Metformin.

The study's lead author, Ronac Mamtani, said he believes the study will help doctors and their patients weigh the potential benefits and risks when selecting between different diabetes medication."

"The risk does seem to be common among both drugs in the TZD class, and the fact that we have compared bladder cancer risk among patients taking each of those drugs provides essential information, because a safety warning on a drug is only useful to a doctor when they have knowledge of the same risks for an alternative drug," Mamtani said. "There are many factors clinicians must weigh in deciding which drug to use to control a patient's diabetes, and these new data provide important information to include in that decision-making process." 

Researchers said that doctors who care for patients with diabetes should be very aware of any bladder-related symptoms patients might be having, like blood in the urine, and take steps to further evaluate those issues.