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The difference between pill users and non-users was small, and the findings do not prove a cause-and-effect relationship, the researchers stressed.
For one, it's known that some women with asthma see their symptoms flare at certain points in the menstrual cycle. Fluctuating hormone levels are suspected to be the reason, explained Nwaru, of the University of Gothenburg in Sweden.
"Anecdotal evidence suggests that some women with menstrual-related asthma appear to get relief by taking hormonal contraceptives," he said.
But, Nwaru added, studies on the question have come to conflicting results.
This latest study, published online Nov. 23 in the journal Thorax, is the "most robust" to date, according to Nwaru. It followed a large group of women over 17 years and found what could be a "small" protective effect of birth control pills, he said.
According to Parikh, there are various lines of evidence that the body's sex hormones (like estrogen) affect asthma: Before puberty, boys are more likely to develop asthma than girls are, but after puberty, the reverse is true. Boys also outgrow asthma more often, which all results in asthma being more common in women than men.
Pregnancy plays a role, too. One-third of pregnant women with asthma see their symptoms improve, while just as many have a worsening, said Parikh, who is also a clinical assistant professor at NYU Grossman School of Medicine in New York City.
Why would hormones matter? Parikh said hormonal fluctuations can affect immune system cells called mast cells, which are involved in allergic reactions and asthma.
To Parikh, it's important for doctors and patients to be aware that hormones can affect asthma control. But the evidence does not support using hormones as treatment, she said.
The current study included 83,084 women aged 16 to 45 who had asthma. At the outset, one-third were using oral contraceptives. Nwaru's team used hospital and prescription records to track how the women fared over 17 years.
In general, the investigators found, women on combined birth control pills (containing estrogen/progestin) were slightly less likely to have severe asthma attacks. "Severe" meant symptoms bad enough to require an ER visit or hospital admission, or treatment with oral corticosteroids.
It's only a small effect, Nwaru said, and more studies are needed to confirm the link.
Like Parikh, he did not see the study as support for using the pill as an asthma treatment.
Instead, Nwaru said, researchers should try to better understand the "biological processes" through which hormonal contraceptives affect asthma.
For now, Parikh encouraged women to talk to their doctor if their asthma symptoms are not well-controlled.
"Often women are caregivers and put their own health on the back burner to take care of others," she said. "So if you're waking up at night with asthma symptoms, easily winded from doing household chores or playing with kids, or using your albuterol more than two times per week -- these are signs your asthma is not controlled and you should see a specialist."
SOURCES: Bright Nwaru, PhD, associate senior lecturer, molecular medicine, Krefting Research Center, University of Gothenburg, Sweden; Purvi Parikh, MD, national spokeswoman, Allergy and Asthma Network, Vienna, Va., and clinical assistant professor, medicine and pediatrics, NYU Grossman School of Medicine, New York City; Thorax, Nov. 23, 2020, online
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