A trial run by Auckland University's bone research group found the supplements caused a 40 percent increase in heart attacks, the Sunday Star-Times reported.
Ian Reid, the professor who directed the study, said women over 70 who had been on the trial were recommended to stop taking extra calcium, especially if they had a history of heart or kidney disease, pending further investigations.
About 1,500 women took part in the trial, which was funded by New Zealand's Health Research Council.
Reid told the paper that three other international studies on calcium supplements had since been reviewed, and all showed varying degrees of elevated risk of heart attacks, ranging from 10 to 20 percent.
"When you put four studies together, you need to take it more seriously," he said. "This is potentially very important because there are so many people around the world who take calcium supplements."
Reid said the study found calcium supplements dramatically reduced older women's risk of bone fractures and bone density loss. "But heart disease kills you, whereas bone disease can, but it isn't quite so lethal."