Magnesium is an essential mineral that is sorely lacking in most people and controls over 350 enzymatic functions. Magnesium is antagonistic to calcium so as you raise levels of calcium, magnesium lowers. Another scenario is as you lose vital magnesium then calcium becomes more prevalent.
There are a number of issues that magnesium controls such as blood pressure, anxiety, low energy, immune system etc. Vitamin D3 works surrealistically with magnesium which should help magnesium absorb and become active, so getting your vitamin d levels checked is important as most are low.
There are a number of different forms of magnesium which can work differently in people so choosing the form is vital. UltraMag from Source Naturals offers a multi blend form of magnesium. Best to stay away from Magnesium Oxide since it is a poorly absorbed and utilized form.
For additional resources about magnesium, search for the Selected Works of Dr. Mildred Seelig.