Calcium supplements may increase the risk of a heart attack in older women, New Zealand research suggests.
Calcium keeps the bones healthy
The supplements are often prescribed to postmenopausal women to help counter loss of bone density.
Previous research suggested they might also protect against vascular disease by cutting blood cholesterol levels.
However, the latest British Medical Journal study found the opposite to be true, although UK experts warned women not to stop taking medication.
The University of Auckland team followed 1,471 healthy postmenopausal women for five years.
Each woman either took a daily calcium supplement, or a dummy pill.
Heart attacks were more common in the group who took the supplements.
After a careful analysis of the data, the researchers confirmed 36 heart attacks in 31 women who took the supplements, compared with 22 heart attacks in 21 women who took the placebo.
Rates of stroke and sudden death were also higher in the supplement group - although not conclusively.
The researchers said the supplements may raise the risk of a heart attack by accelerating hardening of the blood vessels.
Lead researcher Professor Ian Reid said: "It is likely that this is primarily a problem for elderly women because they are more likely than younger subjects to have prevalent coronary heart disease.
"Therefore, it seems wise to advise against calcium supplementation in those over the age of 70 years and in those known to have coronary heart disease.
"Aiming at a total calcium intake of approximately 1g/day - equivalent to four servings of dairy products - seems sensible in these subjects."
Judy O'Sullivan, a cardiac nurse at the British Heart Foundation, said more rigorous research was needed before any firm conclusions could be drawn - particularly as previous research had come up with very different findings.
She said: "Anyone who has been advised by their doctor to take calcium supplements to protect their bones should not stop doing so in light of this study alone without medical advice."
A spokeswoman for the National Osteoporosis Society agreed that more research was required.
She stressed that calcium supplements could help to maintain strong bones, and reduce the risk of fracture, particularly older people whose dietary intake of the mineral was low.
Pamela Mason, nutritionist and spokeswoman for the Health Supplements Information Service (HSIS), which is funded by several leading supplement manufacturers, said the study was small and had a high drop out rate.