Metformin lowered risk for cancer in people with type 2 diabetes

Researchers in the United Kingdom reported that metformin reduced the risk for cancer in patients with type 2 diabetes when compared with patients taking other forms of treatment.

“Fewer than 8% of the cohort of metformin users were diagnosed with cancer during a maximum of 10 years of follow-up compared with 11% of a comparator cohort of non-users,” the researchers wrote.

The observational, historical cohort study included data from 4,085 people aged 35 years or older with type 2 diabetes who were new users of metformin during 1994 and 2003. The researchers compared the data with that of 4,085 people the same age with type 2 diabetes who were never users of metformin.

Patients receiving metformin were younger, had higher mean values of BMI and HbA1c when compared with patients in the comparison group and were more likely to have smoked. More patients in the metformin group were treated with sulfonylureas within the last three months, and fewer patients in this group used insulin within a year when compared with controls.

The researchers calculated HRs for cancer diagnosis and used Cox regression analysis to adjust for baseline characteristics.

During follow-up, 7.3% of patients in the metformin group had a diagnosis of cancer compared with 11.6% of patients in the comparison group. The median time to cancer diagnosis was 3.5 years in the metformin group vs. 2.6 years in the comparison group.

Unadjusted Cox regression analysis indicated a statistically significant decrease in the risk for cancer among new metformin users (HR=0.46). Higher BMI and HbA1c were associated with a decreased risk for cancer which, according to the researchers, “may be a diagnostic bias, with high BMI and HbA1c indicative of less frequent health care-seeking behavior.”

Those who never smoked or stopped smoking had a decreased risk for cancer. An increased risk was found among men, and this risk increased with age.

“We believe there is now a strong case to conduct a randomized trial to establish whether metformin is protective in a population at high risk for cancer,” the researchers wrote. “This study has produced sufficient epidemiological evidence that metformin reduces the risk for cancer to make further investigation a high priority.”

Libby G. Diabetes Care. 2009;32:1620-1625.