The etidronate and calcium combination is used to treat osteoporosis and bone metabolism problems.
The bisphosphonate, etidronate, increases bone turnover shifting the balance between bone formation and resorption towards formation. The calcium is the "raw material" the body uses to form the bone.
Stomach irritation, gas, and constipation may occur. The etidronate may ironically interfere with the way the bone mineralizes and the crystal formation. Brittle bones are the result. It is for this reason that the combination is sometimes given intermittently.
People need sufficient quantities of the raw materials needed to build bones - calcium and vitamin D. It is for this reason the combination makes sense, however even with the calcium content, people may need to take supplementation.
The etidronate ingredient is poorly absorbed and food blocks its absorption. To avoid this interaction, the drug needs to be taken on an empty stomach.
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The relevant product monographs must be regarded as the appropriate sources of prescribing information.