People taking calcium supplements have about a 30% higher risk of heart attack, research out today suggests.
A review of existing studies on some 12,000 people found an increased risk for those on supplements, which are often prescribed to older women for the prevention or treatment of osteoporosis.
People taking supplements equal to 500mg or more per day were analysed through 11 studies, which compared them with people not on supplements.
According to the Food Standards Agency, adults need 700mg of calcium a day, which should come from dietary sources including milk, cheese and green, leafy vegetables.
Today's study, from experts at the University of Auckland and the University of Aberdeen, said diets high in calcium do not increase the risk of heart attacks.
It is the effect of supplements, which increase the levels of calcium circulating in the blood, which causes the increased risk.
Experts believe higher blood serum levels lead to hardening of the arteries, which can cause heart attacks.
The authors said: "Ingestion of equivalent doses of calcium from dairy products has a much smaller effect than calcium supplements on serum calcium levels."
Today's study excluded patients who were taking both calcium and vitamin D supplements. Vitamin D is needed for the body to absorb calcium.
"Given the modest benefits of calcium supplements on bone density and fracture prevention, a reassessment of the role of calcium supplements in the management of osteoporosis is warranted," they said, writing online in the British Medical Journal.