--if one is supplementing with both calcium and magnesium, it's a good idea to take them at different times of day. From a chemical standpoint, both calcium and magnesium are earth metals and though they are taken in compound form they can compete chemically at destination sites in the body if taken at the same time.

Not that this generally causes any bad symptoms or anything, but one may not get the biggest "bang for the buck" out of one's supplement dosage in that scenario.