Long-term metformin use is associated with a decreased risk of colorectal cancer in men with diabetes, research suggests.
Metformin is the first-line treatment for people with type 2 diabetes, and in some cases is prescribed to those with type 1 diabetes. The drug helps to lower blood glucose levels, lower insulin resistance, and has also been linked with lowering the risk of certain cancers.
In this retrospective study, scientists reviewed participants in the Kaiser Permanente Northern California Diabetes Registry between 1994-1996. Participants had to be aged 40 years or older to qualify for the review, and take at least one diabetes medication.
After screening, a total of 47,351 individuals were included in the final cohort, 21,524 of whom took metformin during follow-up.
The results showed a risk reduction for colorectal cancer among those who took higher doses of metformin and within men who had been taking the drug for over five years. The risk reduction in women taking the drug for more than five years was not significant. Overall, with both genders and confounding factors taken into account, there was a reduction in risk bit not to a degree of statistical significance.
Among participants who first took a sulphonylurea before moving to metformin, they too had a decreased colorectal cancer risk.
Writing about diabetes types among the participants, the authors wrote: “It was not possible to differentiate patients with type 2 versus type 1 diabetes and so eligibility was restricted to patients’ age.”
Because only an association was observed, it cannot be stated that metformin reduces the risk of colorectal cancer. The researchers concluded that future cohorts will need to be examined to confirm if this association persists.
They added: “The observed sex difference in the association between long-term use of metformin and reduced colorectal cancer risk remains unexplained and may merit further investigation.”
The findings have been published in the journal Cancer Epidemiology, Biomarkers &Prevention.

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