I don't recall feeling drowsy from Metformin; maybe you are just tired and contributing that to the pill. I sit down to do a puzzle or read and suddenly I am drowsy and doze off: that's all it takes. Back to the MET -- your body will need a bit of time to adjust to the pill, but 500 mg per day is a lower dose; I started on 500 mg per meal, but stopped the one with breakfast as that made me feel sick. Later, I discovered that I had less side effects if I took 1000 mg with my dinner instead.
Sometimes, I would feel a bit nauseated, weak, shakey, and dizzy at first; but then I discovered that my blood sugar levels were a bit low for me. So, I would just have a light snack and that helped; some things helped quicker than others (ie 1/2 cup beer). My doctor recommended an apple but said the beer thing was OK too, if it made me feel better.
If I stick to my plan very well, and take the pills, sometimes I would get those symptoms. As time went on, they lessened. These pills are supposed to help you release excess water, and help your hormones help your body to regulate your blood sugar levels better. My blood sugar levels are now down to the 5's where they should be. Sometimes, if I eat something higher in sugar (ie chocolate) and take the MET, I may feel off.
Some people have more frequent bowel movements and some have diarrhea; and others have to urinate more often. I had two of those; not too much diarrhea unless I was sick or something now, as I am eating more whole grains which helped some. Some say that MET helps you to lose weight but you still have to eat healthy and exercise like anyone else does.