Calcium supplements taken by many older people could be increasing their risk of a heart attack, research shows.
The study, in the British Medical Journal, said people who took supplements were 30% more likely to have a heart attack.
Data from 11 trials also suggested the medicines were not very effective at preventing bone fractures.
Almost 3m people in the UK are thought to have osteoporosis and many take calcium pills to prevent fractures.
The study recommends doctors review their use of calcium supplements for managing osteoporosis.
The National Osteoporosis Society said most people should be able to get enough calcium through their diets, rather than reaching for the medicine cabinet.
The researchers said those who had a diet naturally high in calcium were at no increased danger.
'Limited benefit'
In all 12,000 people aged over 40 took part in the trials of calcium supplements of 500mg or more a day.
The risk of heart attack was seen across men and women, was independent of age and the type of supplement given.
A small increased risk of death was seen in the study but was not statistically significant, the researchers said.
The reason for the increased risk of heart attack is not clear but it is thought the extra calcium circulating in the blood could lead to a hardening of the arteries.
Calcium in the diet is safe and the Food Standards Agency recommends adults have 700mg of calcium a day from milk, cheese and green, leafy vegetables.
Dr Alison Avenell, from the University of Aberdeen which did the research with colleagues in New Zealand and the US, said the evidence suggests calcium supplements only have a limited benefit in preventing fractures, especially when compared to other treatments available.
"It is a balance of risks - people should consider the risks involved and how they apply to their own circumstances and discuss the matter with their GP," she said.
She added the results did not necessarily apply to younger people with conditions for which they take calcium.
Judy O'Sullivan, senior cardiac nurse at the British Heart Foundation, said the results should be interpreted with caution because the trials did not set out to look at the risk of heart attack.
"However, the research should not be completely ignored," she said.
"Any new guidelines on the prevention of fractures in those most vulnerable to them should take this type of analysis into account."
Dr Claire Bowring, of the National Osteoporosis Society, said: "We've always recommended that people should aim to get the calcium they need from their diet to help build stronger bones.
"If you get all of the calcium that you need from your diet and adequate vitamin D from exposure to sunshine, then a supplement will not be necessary."
She said there were still questions to be answered about the treatment of osteoporosis but advised people taking calcium supplements to talk to their GP, especially if they have a heart condition.
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