Women who take antibiotics over a long period of time are at increased risk of heart attack or stroke, a study claims.
The study, published in the European Heart Journal, found that women aged 60 or older who took antibiotics for two months or more had the greatest risk of cardiovascular disease, but long duration of antibiotic use was also associated with an increased risk if taken during middle age (aged 40-59).
The researchers could find no increased risk from antibiotic use by younger adults aged between 20-39.
The possible reason why antibiotic use is linked to an increased risk of cardiovascular disease is because antibiotics alter the balance of the micro-environment in the gut, researchers said.
"Antibiotic use is the most critical factor in altering the balance of microorganisms in the gut. Previous studies have shown a link between alterations in the microbiotic environment of the gut and inflammation and narrowing of the blood vessels, stroke and heart disease," said Lu Qi, director of the Tulane University in the US.
The researchers studied 36,429 women who took part in the Nurses' Health Study, which has been running in the USA since 1976. The current study looked at data from 2004 to June 2012.
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In 2004 the women were aged 60 or older, and they were asked about their use of antibiotics when they were young (20-39), middle-aged (40-59) or older (60 and older).
The researchers categorised them into four groups: those who had never taken antibiotics, those who had taken them for time periods of less than 15 days, 15 days to two months, or for two months or longer.
During an average follow-up period of nearly eight years, during which time the women continued to complete questionnaires every two years, 1,056 participants developed cardiovascular disease.
The researchers found that women who used antibiotics for periods of two months or longer in late adulthood were 32 per cent more likely to develop cardiovascular disease than women who did not use antibiotics.
Women who took antibiotics for longer than two months in middle age had a 28 per cent increased risk compared to women who did not.
These findings mean that among women who take antibiotics for two months or more in late adulthood, six women per 1,000 would develop a cardiovascular disease, compared to three per 1,000 among women who had not taken antibiotics.
"By investigating the duration of antibiotic use in various stages of adulthood we have found an association between long-term use in middle age and later life and an increased risk of stroke and heart disease during the following eight years," said Yoriko Heianza, a research fellow at Tulane University