I was taking magnesium oxide and didn't notice any side effects but just started magnesium complex and notice abdominal and chest discomfort which I'm not a 100% sure it's because of magnesium!
Well what else is in the complex, and how much of it? My guess is it's just fine and any pains are unrelated to the magnesium complex. OTOH, what else are you taking?
It depends if it is prescribed or not. Over the counter medications are more safer than prescribed medications, obviously.