I've had some side affects with Metformin listed below. 

You can search google for metformin side affects to find different sources of information.  Take what you read with a grain of salt, and always check with your doctor. 

Here's a list of side affects that I found on the web.  Again, don't take it as gospel. 

SIDE EFFECTS: The most common side effects with metformin are nausea, vomiting, gas, bloating, diarrhea and loss of appetite. These symptoms occur in one out of every three patients. These side effects may be severe enough to cause therapy to be discontinued in one out of every 20 patients. These side effects are related to the dose of the medication and may decrease if the dose is reduced.

A serious but rare side effect of metformin is lactic acidosis. Lactic acidosis occurs in one out of every 30,000 patients and is fatal in 50% of cases. The symptoms of lactic acidosis are weakness, trouble breathing, abnormal heartbeats, unusual muscle pain, stomach discomfort, light-headedness and feeling cold. Patients at risk for lactic acidosis include those with reduced function of the kidneys or liver, congestive heart failure, severe acute illnesses, and dehydration.