Taking birth control pills may cut the risk of severe bouts of asthma in women of reproductive age with the respiratory condition, say researchers.

The findings, published in the journal Thorax, indicate that the observed protective effect is relatively small, and doesn't include progestogen-only hormonal contraceptives. "Female sex hormones are thought to partly explain the clear differences in the incidence and severity of asthma between the sexes," said study authors from the University of Gothenburg in Sweden. "And fluctuations in levels of these hormones during the menstrual cycle have been linked to worsening asthma symptoms in some women," they wrote.

While the potential impact of synthetic sex hormones on asthma in women has been repeatedly studied over several decades, no consensus has yet been reached. In a bid to clear up the uncertainties, the researchers explored the potential impact of different types and periods of use of hormonal contraceptives on asthma severity and what influence weight (BMI) and cigarette smoking might have. They drew on information entered into the Optimum Patient Care Research Database (OPCRD) to find women of reproductive age (16-45) who also had asthma.

OPCRD is a large population-based, long term, anonymised database of 630 primary care practices across the UK, containing the health records of more than six million patients. Hospital admissions, emergency care department visits, and prescriptions for asthma treatment were tracked from the start of 2000 until the end of 2016 for a total of 83, 084 women to gauge changes in the severity of the condition. Previous and current hormonal contraceptive use (combined oestrogen/progestogen and progestogen-only) for periods of 1-2 years, 3-4 years, or 5 plus years was compared with no use at all.

At the start of the study, around a third (34 percent) of the women were using hormonal contraceptives: 25 percent combined and nine percent progestogen-only. The proportion of women who had bouts of severe asthma rose with increasing age and BMI and a higher number of previous pregnancies. The findings showed that previous and current use of any and combined hormonal contraceptives was associated with a lower, albeit relatively small, risk of severe asthma bouts compared with no use at all. And while the use of hormonal contraceptives for one-two years didn't affect risk, use for three-four years and for five or more years was associated with a lower risk compared with no use at all. "It's still not clear how synthetic sex hormones might affect asthma, further research will be needed to explore the underlying biological processes," the team noted.