Metformin is usually the first drug prescribed after patients are diagnosed with diabetes, lifestyle modifications are not effective and A1C of 9 and below are experienced. It is widely used because it is safe, effective and inexpensive.

Metformin reduces glucose production in the liver and decreases insulin resistance, thus allowing glucose to be absorbed and used in the muscle and liver tissues. Metformin can be used alone (monotherapy) or combined with other anti diabetic drugs. It is available as immediate release tablets and extended release tablets. Metformin is usually started at a low dose and increased as tolerated. Dosage forms available include immediate-release tablets 500mg, 850mg and 1000mg; extended release tablets 500mg, 750mg and 1000mg; oral solution 100mg/ml and extended release oral suspension reconstitutes to 500mg/ml.

About 30% of patients taking metformin report upset stomach and/or diarrhea. Sometimes these side effects decrease over time especially if taken with meals. Metformin taken long-term may cause a decrease in B12 levels.