Use of metformin in people aged ≥60 years with type 2 diabetes mellitus (T2DM) is associated with a reduced risk of developing dementia, according to a study that included 14,558 people in Taiwan. The risk was reduced relative to that in people in the same age group who had T2DM but were not receiving metformin. A dose–response relationship was seen, such that higher daily and cumulative doses of metformin were associated with a lower risk of dementia. The findings suggest that metformin could protect cognitive function in this patient population, and an understanding of the mechanisms could identify novel targets for prevention of dementia more widely.