The main side effects are plumbing upsets including; nausea, diarrhea, bloating and gas. It can also cause Lactic Acidosis as I know to my cost. And sadly, no, it won't help you lose weight, sorry about that.
Most people get over the initial side effects (except the gas) within a couple of weeks of starting the drug, but it can recur for a while if the dosage is increased. Hope that helps.