When you buy magnesium, look at the list of ingredients.
Magnesium oxide is not absorbed well, but it is cheap, so manufacturers use it.
is bulky, so they cannot fit much in a multivitamin pill.
I suggest these.
Source Naturals, Ultra-Mag, with magnesium citrate, taurinate, malate, glycinate and succinate.
Peter Gillham's Mag Calm