New US research has suggested that women who take antibiotics over a longer period of time may have an increased risk of death from heart disease and other causes.

Carried out by researchers from Tulane University School of Public Health and Tropical Medicine and Harvard T.H. Chan School of Public Health, the study looked at 37,510 women aged 60 years and older who were all free from heart disease and cancer at the start of the investigation.

The women were asked to report on their antibiotic use, before being classified into groups based on how long they had used antibiotics: not at all, less than 15 days, 15 days to less than two months or two or more months.

After following the women for eight years between 2004 and 2012, the team found that those who took antibiotics for two months or longer in late adulthood were 27 percent more likely to die from all causes during the study than women who did not take antibiotics.

In addition, taking antibiotics for two or more months was also associated with a 58 percent higher risk of death from cardiovascular disease when compared to no antibiotic use.

The results still held true even after researchers took into account other lifestyle factors such as diet, obesity, and medication use.

The researchers also found that the link between antibiotic use in late adulthood and an increased risk of death was stronger among women who had also used antibiotics in middle adulthood, from ages 40 to 59 years.

While previous studies have found antibiotic use is associated with long-lasting changes to microorganisms that live in the human gut, known as gut microbiota, researchers had yet to examine relatively healthy populations of people to determine how duration of antibiotic use at different times during adulthood might be related to all-cause and cause-specific risk of death, according to study author Lu Qi, M.D., Ph.D., professor of epidemiology at Tulane University in New Orleans.

"Gut microbiota alterations have been associated with a variety of life-threatening disorders, such as cardiovascular diseases and certain types of cancer," Qi said. "Antibiotic exposure affects balance and composition of the gut microbiome, even after one stops taking antibiotics; so, it is important to better understand how taking antibiotics might impact risks for chronic diseases and death."

No significant association was found between antibiotic use and death from cancer.

"Although we observed a notable association between long-term antibiotic use and risk of death, it isn't yet clear whether long-term antibiotic use is the specific cause of the association. For example, women who reported antibiotic use might be sicker in other unmeasured ways," Qi said. "These results, however, contribute to a better understanding of risk factors for all-cause and cardiovascular death. We now have good evidence that people who take antibiotics for long periods during adulthood may be a high-risk group to target for risk-factor modification to prevent heart disease and death."

According to the World Health Organisation (WHO), antibiotic resistance is one of the biggest threats to global health, and is in part caused by the misuse and overuse of antibiotics. Each November WHO organizes Antibiotics Awareness Week to provide information and advice on how to safely use the medication and help prevent antibiotic resistance.

The preliminary research was presented at the American Heart Association's Epidemiology and Prevention / Lifestyle and Cardiometabolic Health Scientific Sessions 2018, taking place March 20 to 23 in New Orleans, Louisiana.