It is generally recommended that people not use metformin and alcohol at the same time. Taking metformin and alcohol together can increase your risk of developing a life-threatening condition called lactic acidosis. Talk with your PCP if he/she permits "small amounts" of alcohol while taking metformin. If he/she does, you better have explained what "small amounts" means to them before you blindly consume too much alcohol.
Alcohol will lower your blood sugar as sk123 has said. I take metformin and insulin and when I decide to have an alcoholic beverage, I usually don't take my metformin that evening, of course, you should ask your Dr before drinking alcohol...bottoming out (bs going to low) is no fun!!
According to the MedHelp drug page on metformin, alcohol lowers blood sugar and may increase the risk of lactic acidosis while you are taking this medicine.
Here's the link:
http://www.medhelp.org/drugs/metformin/show/4596
You'll see this under interactions.