Metformin has been shown to have pro-longevity and healthspan extending properties in the roundworm Caenorhabditis elegans, mice, and humans. In other model organisms such as Drosophila (fruit fly) and rats, similar benefit has not been identified. Although data from prospective clinical trials in humans on metformin in aging are only just planned or beginning to emerge, widespread use of the drug in aging in otherwise healthy individuals requires far more granular understanding of its effects, and the genetic and environmental determinants of its success in promoting aging versus potential detrimental effects. T2D, type 2 diabetes.