Hi Minoucha,
Metformin on its own cannot cause a hypo. However, alcohol has the effect of lowering blood sugar, so drinking alcohol when you are on metformin could cause a hypo. If you regard the guideline as 2 units a day, you should be pretty safe. If you have your week's 14 units in one evening then a hypo is almost a certainty.
Type 1s are more susceptible to hypos than type 2s. I am type 2 and don't wear a medical band, but then I monitor my blood sugars regularly and drink in moderation. If your blood sugars drop into the hypo range (usually below 4 but can vary from person to person) you will get plenty of warning signs - feeling dizzy, disoriented, sweating, hands shaking, becoming aggessive and suddenly argumentative.