The study identified 1,340 consecutive type 2 diabetic outpatients (mean age 63, 56% female). Participants had no history of cancer and all were started on insulin therapy between 1998 and 2007 in Florence, Italy. Incident cancer diagnoses were identified using hospital admission data or the Mortality Register (for cancer deaths).
112 patients were identified with incident cancer (29 gastrointestinal, 16 lung, 14 pancreatic, 53 others) over a media follow up of 73.9 months (approximately 6 years). These cases were matched to controls for age, gender and BMI at insulin initiation from a cohort of 370 patients. In those patients who were prescribed metformin that was a significantly reduced risk of cancer (odds ratio 0.46, 95% CI 0.25-0.85, p=0.014).
The authors conclude that, "the reduction of cancer risk could be a further, relevant reason for maintaining metformin in insulin-treated patients ".
The authors note that there are limitations in the data and therefore the analysis. Prescription bias could not be ruled out and some patients may have been misdiagnosed as diabetic. Also, the greater incidence of cancer in those patients that did not receive metformin may be due to co-morbidities.
Action: Clinicians should be aware of this small study. The results of this study may be useful when discussing the risks and benefits of metformin treatment.
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