Calcium supplements, taken by millions of elderly people and post-menopausal women to prevent bone thinning, may double the risk of having a heart attack, a study has found.
Researchers warned that the pills should be "taken with caution", and experts commenting on the findings questioned their safety.
Previous studies linked higher calcium intake with a reduction of heart disease risk factors such as high blood pressure, obesity and Type 2 diabetes.
But the new research from Germany points to a vital difference between dietary calcium from sources such as milk, cheese, greens and kale, and supplements.
Taken in supplement form, the mineral floods the bloodstream, causing changes that may produce hard deposits on the walls of arteries, scientists believe.
Researchers analysed data on 23,980 German men and women aged 35 to 64 taking part in a study called the European Prospective Investigation into Cancer and Nutrition.
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Over a period of 11 years, a total of 354 heart attacks, 260 strokes and 267 associated deaths were recorded.
Participants whose diets included a moderate intake of calcium - around 820 milligrams (mg) daily - from all sources had a 31% lower heart attack risk than those with the lowest intake.
But no significant benefit was seen when calcium intakes rose to more than 1,100 mg per day.
The picture changed for the worse when the scientists focused on supplements. People taking supplements that included calcium were 86% more likely to suffer a heart attack than those taking no supplements.
For participants who only used calcium supplements, heart attack risk more than doubled.
The researchers led by Professor Sabine Rohrmann, from the University of Zurich, Switzerland, wrote in the online edition of the journal Heart: "In conclusion, this study suggests that increasing dietary calcium intake might not confer significant cardiovascular benefits, while calcium supplements, which might raise MI (myocardial infarction, or heart attack) risk, should be taken with caution."
In a comment article accompanying the research paper, two experts from New Zealand examined the safety of calcium supplements.
Professors Ian Reid and Mark Bolland, from the University of Auckland, pointed out that previous research had linked the supplements to kidney stones and gut and abdominal symptoms.
For many healthy middle-aged women, taking calcium supplements to ward off brittle bones, the overall protective effect was only about 10%.
Profs Reid and Bolland stressed that dietary calcium, taken in small amounts spread throughout the day, was absorbed slowly.
But supplements caused calcium levels in the blood to soar above the normal range, possibly increasing the risk of artery calcification.
"It is now becoming clear that taking this micronutrient in one or two daily boluses is not natural, in that it does not reproduce the same metabolic effects as calcium in food," wrote the experts, from the University of Auckland's faculty of medical and health sciences.
"The evidence is also becoming steadily stronger that it is not safe, nor is it particularly effective. Therefore, the bolus administration of this micronutrient should not be encouraged, rather, people should be advised to obtain their calcium intake from an appropriately balanced diet."
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The British Heart Foundation urged people not to stop taking calcium supplements prescribed by their doctors.
Natasha Stewart, the charity's senior cardiac nurse, said: "This research indicates that there may be an increased risk of having a heart attack for people who take calcium supplements. However, this does not mean that these supplements cause heart attacks.
"Further research is needed to shed light on the relationship between calcium supplements and heart health. We need to determine whether potential risks of the supplements outweigh the benefits calcium can give sufferers of conditions such as osteoporosis.
"If you have been prescribed calcium supplements, you should still keep taking your medication, but speak to your doctor if you have any concerns."
Dr Carrie Ruxton, from the Health Supplements Information Service, which is funded by an association representing supplement manufacturers, criticised the research.
She pointed out that calcium intakes were only measured once, and no attempt was made to observe the effect of modifying them or make comparisons with a "dummy" placebo supplement.
She said: "Vitamin and mineral supplements were never intended to treat chronic health conditions. Instead, their role is to help people meet recommended nutrient intakes.
"According to the UK National Diet and Nutrition Survey, women consume only 740mg of calcium per day from food sources on average.
"This is 43mg lower than in 2000/01. Osteoporosis (brittle bone disease) is a real issue for women and it is irresponsible for scientists to advise that women cut out calcium supplements on the basis of one flawed survey, particularly when the link between calcium, vitamin D and bone health is endorsed by the European Food Safety Authority."