Pulled this from my vitamins thread:
Magnesium (oxide, taurate, citrate)
Dosage: Minimum 420 - 1000 mg
Info: Aids in the formation of bones & teeth, assists in absorption of calcium & potassium, helps prevents depression, dizziness, muscle twitching, calcification of soft tissue(s), cardiovascular disease, osteoporosis, certain cancers, & possibly reduce cholesterol. Known as a natural relaxant.
There is a few different forms of magnesium, oxide is the most common form and cheapest but doesn't absorb very well. A step up would be citrate but like calcium it is more expensive to produce and can worsen acid reflux and heartburn if you are already prone to getting it. The highest absorbed and most expensive to produce would be the taurate form.
Assimilates best with: Calcium, Phosphorus, B-Complex, Potassium, and Vitamin C & D.
Antagonist(s): Alcohol, Diuretics, high levels of Zinc, Vitamin D & Calcium, excretion of any fluids, foods high in oxalic acid, any fat soluble Vitamins, Flouride.
As stu said it won't absorb as effectively if at all in some cases. Also depletion of other vitamins can occur from certain forms of other vitamins/minerals etc.