I just learned from my doctor that calcium and magnesium compete with one another for the same receptors in the body. I looked this up online and found several articles explaining this. Most people need both, especially for bone health. So it's best to take calcium at one time of day and magnesium at another time. And definitely don't buy the Cal-Mag supplements with both in the same capsule! Our health food store has half a shelf of Cal-Mag supplements.