Re: Metformin : How long before side effects start ? Hey, humph. That's very interesting, because my chemist also has a new supply. The round Metformins were great, but since I've gone onto the long ones, my daily readings have gone up and my HbA1C has gone from 6.1 to 7.0 My GP thought it was possibly stress or anxiety but I reckon that it may well be the new tablets. I'm going to have to see both my GP and my pharmacist.