Metformin - I take four 500 mg metformin at bedtime and have disabling IBS-D every single day...

Diarrhea is the #1 cause of people going off of metformin. The general advice we patients get is to take metformin with food. Not just a couple of crackers but something more substantial, like a sandwich at least. It took a couple of weeks, but that worked for me.

If that doesn't work for you, or if you get more severe side effects, then you should speak to your doctor. He knows your full case and can recommend other approaches.

I hope that helps.