Magnesium competes with calcium for absorption and can cause a calcium deficiency if calcium levels are already low.
Source: Magnesium | University of Maryland Medical Center http://umm.edu/health/medical/altmed/supplement/magnesium#ixzz387XpuCga
Strontium rebuilds bone lost due to osteoporosis and osteopenia without the negative side effects associated with many prescriptions, such as Fosamax, Boniva,and Actonel. Strontium is available in several forms, including citrate, lactate, carbonate, gluconate, and ranelate. The latter requires a prescription and is not available in the U.S. The other forms are available from health-food stores and on-line distributors. Strontium citrate is the most common form and is easily assimilable.