Hi Sugarmog, here’s some info on Metformin, including how it works:
https://www.nhs.uk/medicines/metformin/
as
@Goonergal says it has a cumulative effect so couldn’t be used to counteract a more carby meal. I have taken 3 x 500mg per day since diagnosis and apart from a trial dropping to two which I abandoned as my weight loss stalled, I have taken 3 ever since. During the trial of 2 per day, for that three months my HbA1c only went up from 34 to 35 so negligible difference.
Edit for typo