I was diagnosed with Type 2 diabetes in April 2003 and metformin was prescribed for me some 10 years later.
Metformin has multiple effects:
1) decreases the amount of glucose that the body absorbs from food.
2) promotes the excretion of blood glucose from the large intestine into the stool.
3) reduces the production of glucose in the liver ie. inhibits hepatic gluconeogenesis (which results in the breakdown of lactate, proteins, lipids plus other substrates and formation of gluconeogenic glucose. This provides glucose when dietary intake is insufficient or absent, for instance, overnight).
4) stimulates glucose uptake in skeletal muscle and improves peripheral insulin sensitivity.
Unlike most anti-diabetic drugs, metformin does not stimulate the production of insulin. It lowers blood glucose gradually when combined with a healthy diet and exercise.
IME, it takes 5-6 days before its effects start to be felt and works best following exercise the previous day.