Danish national guidelines recommend discontinuation of metformin 48 h prior to general anaesthesia due to the presumed increased risk of lactic acidosis. By reviewing recent studies concerning the risk of metformin-associated lactic acidosis it is found, that studies indicate, that metformin does not increase the risk of lactic acidosis. However, comorbidities such as cardiovascular insufficiency, sepsis, dehydration and impaired kidney function are risk factors. New guidelines propose discontinuation of metformin on the day of surgery. Patients with Type 2 diabetes and comorbidities should have the levels of arterial pH and lactate monitored.