A commonly prescribed diabetes drug increases the risk of bladder cancer, research has found.
Patients who use pioglitazone, a drug for type 2 diabetes, every day for more than two years double their chances of developing the disease.
The medication for controlling blood sugar levels is known to increase the risk of heart failure, but the European Medicines Agency decided to keep it on the market.
Researchers based in Canada analysed the medical records of more than 115,000 people across the UK who were given the drug over an 11-year period and found there were up to 137 extra cases of bladder cancer per 100,000 person years.
The results, published in the online medical journal bmj.com, show that 470 patients were diagnosed with bladder cancer during the average 4.6 years of follow-up, a rate of 89 per 100,000 person years. The rate in the general UK population aged at least 65 is 73 per 100,000 person years.
The analysis involved 376 cases, matched to 6,699 controls.
If patients had ever taken pioglitazone they were at an 83% increased risk of bladder cancer, which corresponds to 74 per 100,000 person years.
This increased to 88 per 100,000 person years for patients who had taken the drug for two years or more and 137 per 100,000 years for patients who had taken 28,000mg or more.
Pioglitazone is similar to another drug, rosiglitazone, which also increases the risk of heart failure but does not increase the chance of bladder cancer.
Dominique Hillaire-Buys and Jean-Luc Faillie, from the Department of Medical Pharmacology and Toxicology in Montpellier, France, said on bmj.com: "It can confidently be assumed that pioglitazone increases the risk of bladder cancer. It also seems that this association could have been predicted earlier.
"Prescribers who are ultimately responsible for therapeutic choices can legitimately question whether the benefit-risk ratio of pioglitazone is still acceptable for their patients with diabetes."
The data used was from the General Practice Research Database (GPRD), which contains anonymous patient records from more than 600 UK general practices.
Researchers studied 115,727 patients newly treated with diabetes drugs from 1988 to 2009. Cases of bladder cancer were identified and matched to up to 20 healthy control patients.