I don't know about shitting your pants levels bad but gastrointestinal side effects are very common with Metformin, on the order of ~30% taking it will have... quite a time at the start (I was definitely one). My doctor said something like 10%-15% of diabetics end up having to move to something else because while it generally gets better for most after taking it a while, it doesn't for all. On the flipside, many take it and would never even guess that could be a side effect. Just luck of the draw.
