A new large-scale study has found that long-term metformin use does not help slow down cognitive decline in diseases of the aging brain. The findings are of importance because diabetes is a risk factor for a number of neurodegenerative diseases, and metformin, the first-line drug treatment for type 2 diabetes, may affect an important aspect of cognitive impairment in older adults. Scientists knew that having decreased insulin sensitivity negatively impacts memory formation and prevents insulin from doing its job, including preventing the build-up of plaque in Alzheimer's disease. Some studies hinted that short-term use of metformin might actually protect from cognitive impairment as the treatment helps correct insulin issues and promotes the formation of new neurons. This new research, however, suggests that this protective effect from metformin may be true only for a limited period of time. After following a total of 9,300 patients with type 2 diabetes for 12 years, Taiwanese researchers at Taipei Medical University found that long-term metformin increased the risk of both Parkinson's and Alzheimer's disease. The study showed that the longer a patient used metformin, here for more than 300 days and at doses greater than 240g, the higher the odds of developing these diseases later in life. In fact, the risk for Parkinson’s disease or Alzheimer’s dementia went up over 50 per cent during a 12-year period in those who took metformin when compared to those who did not. The higher incidences of Parkinson’s and Alzheimer's dementia were still significantly higher for participants on metformin after adjusting for age, gender and diabetes severity. However, it was not known whether other diabetes drugs, like insulin or sulphonylureas, used by some of the participants compounded those risks. The researchers were nonetheless surprised by the findings as not too long ago, a 6,000-patient study of metformin's cognitive effects, conducted at Tulane University, found the opposite - that the longer a patient used metformin, the lower the chances of developing cognitive impairment. In light of these seemingly conflicting findings, further large scale studies are now needed to give a clear answer as to whether metformin really helps or not. The findings were presented at the 13th International Conference on Alzheimer's and Parkinson's Diseases.