Very safe. Some people have tummy problems with it--diarrhea, gas, stomach pain, etc. For those there is a time-release version that seems to work better.
When you are first diagnosed diabetic, metformin is the first drug your doctor starts you on, for blood sugar control. I understand it also helps you lose a little weight, at first. If it doesn't work, then they put you on something stronger, and maybe more dangerous. There are lots of new diabetes drugs coming out these days because diabetes is getting so popular.