After - so long as I remember. When I forget (which is a fairly regular occurrence
) I take it and have something like a few nuts with it, to cushion the impact if it's a while since I've eaten. If I
really forget (i.e. it's only a couple of hours until my next tablet is due) I don't bother to take it, and just take the next one when the time arrives. I don't think metformin is as time-critical as some drugs and it's not the end of the world if you miss the occasional dose.
Incidentally, anyone got a fool-proof way of remembering to take tablets? I draw the line at phone alarms and such