This trial indicates metformin suppresses gains in strength and muscle mass associated with resistance training in elderly patients. Its results support previous research suggesting the drug suppresses the benefits of resistance training in multiple populations. In patients who do not require immediate improvements in glycemic control, metformin’s suppression of exercise-induced muscle strength and size gains could increase risk of disability and mortality. These risks may provide an argument for not prescribing metformin to elderly patients. In diabetic and prediabetic patients, it is worth considering non-pharmaceutical treatments that lead to improved glycemic control without reducing the benefits of exercise.