"Lactic acidosis is rare, but it is a serious side effect. It’s caused by a buildup of lactic acid in your blood. Lactic acid is a chemical that is naturally produced by your body as it uses energy. When you take metformin, your body produces more lactic acid than it usually does. When you drink alcohol, your body can’t get rid of lactic acid as quickly. Drinking too much alcohol, especially with metformin, can cause a buildup of lactic acid. This buildup can cause serious damage to your kidneys, lungs, heart, and blood vessels. If lactic acidosis is not treated right away, it can cause these organs to shut down, which can lead to death"
https://www.healthline.com/health/diabetes/metformin-alcohol
I still have the add dram of whisky or glass of wine now and then though