On Metformin 1000 mg with dinner for a couple of years - no problem. Recently (2weeks) increased by a hospital internist to 500 mg with each meal, so 3x daily for a total of 1500 mg. After a week of explosive flatulence and burping (which I was told would pass), had an excruciating day of GI pain and nausea. Controlled by Pepto Bismol and slept for 24 hours. Dropped back to original Metformin dosage of 1000 mg at dinner, as well. Now, two days later, I am passing thick, sticky, totally black stool. What s going on? Should I be concerned?