It’s the type/formulation as much as the quantity. Some of the cheap stuff (magnesium oxide) in the supermarket or local pharmacy or Target or Wal-mart is hard to absorb. It can cause digestive issues. Other formulations can cause heartburn or other problems too.
If you try a formulation and it works for you, you may try 2-4 smaller doses throughout the day to maintain a constant level. Consult with your doctor and your body.
This formulation works for me, and causes no side effects for me. I settled on it after trying several others. Your mileage may vary.