Hi Jill, welcome... Yes magnesium is an essential nutrient we should all be getting through our diet. The ratio of calcium to magnesium is 2:1. If you were getting 1,000 milligrams of calcium then your magnesium should be about half of that.
When we calculate our vitamins we should start by adding up what we get from our diet and then add what ever you need orally through supplement to total the Recommended Daily Alowance (RDA) for each nutrient. Here's an article on the recent Institute of Medicine's report on Calcium and D. This report has been hotly debated, because many feel that we should get more D than it recommends.
Talk to you Dr to see what he/she recommends for you. If your diet is rich in calcium and magnesium you "may" not have to supplement with oral tabs, but you'd need to know your totals first. Here's a list from the National Institute of Health on magnesium.
I hope this helps, and if you have another question don't hesitate to ask.
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