@dbr10 Metformin is (in some CCGs and clinics) offered to non diabetics with polycystic ovary syndrome. I am sure there must be other uses for it too...

It isn't an exclusively D drug.
Regarding Metformin when on very low calorie diets, I am sure I recently came across advice that Metformin should be discontinued with both fasting and very restricted calorific intake - but I am sorry, I have no recollection of where I saw that.
So I went looking. Didn't find the info I was looking for, but I did find this, which is generally informative about Metformin
https://www.drugs.com/metformin.html
and this, which is one of the small-print inserts into the Metformin package
http://packageinserts.bms.com/pi/pi_glucophage.pdf
it is a fascinating read, and this is what it says about Metformin (glucophage) on restricted caloric intake:
Hypoglycemia—Hypoglycemia does not occur in patients receiving GLUCOPHAGE or GLUCOPHAGE XR alone under usual circumstances of use, but could occur when caloric intake is deficient, when strenuous exercise is not compensated by caloric supplementation, or during concomitant use with other glucose-lowering agents (such as sulfonylureas and insulin) or ethanol. Elderly, debilitated, or malnourished patients, and those with adrenal or pituitary insufficiency or alcohol intoxication are particularly susceptible to hypoglycemic effects.
@Henmawson - it is probably best you check it with your doc or nurse.