A study is raising new questions about the safety of calcium, which many women take to protect their bones.

An analysis of data collected from more than 16,000 women who participated in the landmark Women’s Health Initiative found that those who started taking calcium as part of the study were at increased risk for heart attacks and strokes.

The federally funded Women’s Health Initiative is the big study that, among other things, stunned doctors and women in 2002 when it determined that the risks of taking hormones for menopause outweighed the benefits.

When the data from that study were originally analyzed, it found no increased risk for heart problems among women taking calcium and Vitamin D. But most of the women in the study were already taking calcium on their own, which may have hidden any risks.

So Ian Reid of the University of Auckland and his colleagues reanalyzed the data to try to take that into account. The new analysis of data from 16,718 women, published in the British medical journal known as BMJ, found that the women who were not taking calcium when the study started but began taking it when they got into the research project were at 13 to 22 percent increased risk. The risk occurred regardless of whether the women were taking calcium alone or combined with Vitamin D, the researchers found.

The researchers also analyzed data from 13 other studies involving 29,000 people all together, and found increases in the risk for heart attacks and strokes among those taking calcium.

The researchers speculate that there may be something about suddenly starting calcium that boosts the risk, perhaps by causing calcification, or hardening, of the arteries. Calcium may also make it more likely that blood clots will form, they said.

“These data justify a reassessment of the use of calcium supplements in older people,” the researchers wrote.

In an editorial accompanying the analysis, however, Bo Abrahamsen of the Gentofte Hospital in Copenhagen, Denmark, and Opinder Sohota of Queen’s Medical Centre in Nottingham, England, argue that there is still too little information to know for sure one way or the other whether calcium boosts the risk. More research is urgently needed to clarify the issue, they say.

“Clearly further studies are needed and the debate remains ongoing,” they wrote.