Metformin isn't an instant sort of med, so there wouldn't be any advantage to taking it 20 minutes before a meal. You would be more likely to get tummy troubles with it if taken without food.
Hi. If you have the standard version you would normally take it with food and it's effects lasts for a few hours. If you take the SR (Slow Release) version it will act over a 24 hour period so it is less critical when you take it but obviously follow your prescription or leaflet guidelines.
The leaflet says wi8th or soon after a meal .I find if I take my met in the middle of my meal sort of burying it in food I dont have the stomach probs
CAROL