<human>:  What are the side effects of metformin?
<bot>: Common side effects of metformin include digestive issues such as nausea, vomiting, gas, bloating, diarrhea, and an unpleasant metallic taste in the mouth. It can also cause an upset stomach, headache, blurred vision, and trouble breathing. Other more serious side effects include low blood sugar, joint pain, and a decrease in vitamin B12 levels.