Calcium supplements dramatically raise the risk of dementia in women who have suffered a stroke, a study has found.
The pills, taken by thousands of women in the UK to stave off osteoporosis after the menopause, were linked to a seven-fold increase in the chances of developing dementia in old age.
In women with no stroke history whose brains showed signs of less serious damage caused by impaired blood flow, taking calcium tripled the risk.
There was no association between calcium supplements and dementia among women who had no evidence of cerebrovascular disease.
Lead scientist Dr Silke Kern, from the University of Gothenburg in Sweden, said: "Osteoporosis is a common problem in the elderly. Because calcium deficiency contributes to osteoporosis, daily calcium intake of 1000 to 1200mg is recommended.
"Getting this recommended amount through diet alone can be difficult, so calcium supplements are widely used.
"Recently, however, the use of supplements and their effect on health has been questioned."
The observational study did not show that dementia was triggered by calcium supplements, she pointed out.
A total of 700 women aged between 70 and 92 were recruited to take part in the research, none of whom had dementia. Of these, 98 were taking calcium supplements at the start of the study and 54 had already experienced a stroke.
The women were given tests of memory and thinking skills and their progress was followed for five years. Computed tomography (CT) brain scans were also carried out on 447 participants.
During the course of the study, published in the journal Neurology, 54 more women had strokes and 59 went on to develop dementia.
The researchers found that overall, women who took calcium supplements were twice as likely to develop dementia as those who did not.
Further analysis showed that the increased risk was confined to women with a history of cerebrovascular disease. Those who had suffered a stroke were nearly seven times more at risk of dementia if they took calcium supplements.
Women whose CT scans showed "white matter lesions" - areas of brain damage linked to cerebrovascular disease - were three times more at risk if they used the pills.
Dr Kern stressed that the study was small and more work was needed to confirm the findings.
Previous research has suggested that calcium supplements affect the body differently from calcium obtained from food and may be linked to a higher risk of artery disease.
Dr Doug Brown, director of research at the Alzheimer's Society charity, said: "While this research does not show a direct link between calcium supplements and increased dementia risk, it does warrant further investigation.
"People should not worry about eating and drinking calcium as part of a normal, healthy balanced diet.
"Calcium is essential to build strong bones and teeth and also aids muscle contraction.
"This study looked at calcium supplements only, which have a different effect in the body to dietary calcium.
"If you are taking calcium supplements and are concerned, speak to your GP before making any changes to your medication."
Dr David Reynolds, from Alzheimer's Research UK, said: "Calcium is an important part of a balanced diet and while it's known to prevent rickets and osteoporosis, some research has linked calcium supplements to a higher risk of vascular disease.
"This small study goes a step further, linking calcium supplements to a higher dementia risk in women with poor cardiovascular health.
"The finding warrants further exploration, but due to the observational nature of this study and the small number of women involved, it is difficult to draw strong conclusions about the reasons behind the link."
He pointed out that calcium could be readily obtained from dairy products such as milk and cheese, and green leafy vegetables.
"The majority of people should not need calcium supplements," he added.