A major study of around 260,000 women shows that taking the progesterone-only contraceptive pill can increase asthma attacks in some women. The research, published in ERJ Open Research [1], shows an increase in asthma attacks in women with asthma who are taking the progesterone-only pill and either aged under 35, or using fewer asthma treatments, or with a type of asthma called eosinophilic asthma.

There was no increase in asthma attacks for women taking the combined oestrogen and progesterone contraceptive pill.

The researchers say their finding could be an important step towards understanding why women are more likely to have severe asthma than men.

The study was led by Dr Chloe Bloom, a clinical senior lecturer in respiratory epidemiology at the National Heart and Lung Institute, Imperial College London, UK, and funded by Asthma + Lung UK. She said: “Asthma is common in women, and unfortunately, women are twice as likely to die from asthma as men. To help prevent these deaths, we need a better understanding of why women are at greater risk.

“One theory is that sex hormones play a major role. But studying their effects can be tricky. For example, it's often hard to know exactly when someone has reached puberty or menopause. Instead, we can study women who take sex hormone medications like the contraceptive pill, because we know exactly when they started and stopped using them.”

The researchers used the UK’s Clinical Practice Research Datalink to study a group of 261,827 women aged between 18 and 50 who had been diagnosed with asthma. They compared women who had never used the contraceptive pill with women who started taking either the combined or progesterone-only pill. They gathered data on whether the women experienced any asthma attacks – meaning they were either prescribed oral steroids for asthma, they visited the hospital emergency department for asthma or they died of asthma – between 2004 and 2020.

“We found that the combined pill did not affect whether women had asthma attacks. But some women who took the progesterone-only pill had more asthma attacks. These included women under 35 years old, those who used fewer asthma medications, like inhaled or oral steroids, and those with higher levels of inflammation linked to asthma in their blood,” Dr Bloom explained.

The risk of asthma attacks in women under 35 was around 39% higher in those taking the progesterone-only pill; in women taking fewer asthma treatments, the risk was around 20% higher; and in women with eosinophilic asthma (where sufferers have high levels of blood cells called eosinophils that cause inflammation) the risk was around 24% higher.

Dr Bloom said: “Our study is the first to look at a large group of women over time with such detailed data on both their contraceptive pill use and their medical history. We also used a special type of study design that mimics the methods of a clinical trial, but applied to real-world health data. This approach helped us identify which groups of women might be more at risk of asthma attacks while using the pill.

“Asthma is common in women of reproductive age, many of whom are taking the pill. This study helps women and healthcare professionals make more informed decisions about which contraceptive pill might be best for them. The findings also add another piece to the puzzle of why women may be more likely to have severe asthma than men.

“This is an area that hasn’t been studied much. Most research has focused on oestrogen and testosterone, not progesterone. Our study is one of the first to show that progesterone may play an important role. We’re now calling for more research into how progesterone affects asthma, including how it might interact with asthma medications like steroids.”

The researchers are continuing to study the effects of sex hormones on asthma by looking at the possible effects of hormone replacement therapy and at the impact of pregnancy.

Professor Apostolos Bossios is head of the European Respiratory Society’s group on airway diseases, asthma, COPD, and chronic cough, based at Karolinska University Hospital, Stockholm, Sweden, and was not involved in the research. He said: “We know that women are more likely to have asthma and to suffer more severe asthma. We need much more research to understand why asthma is worse for women than men so we can begin to reduce the risk. This major study in an important step towards that aim.

“Women with asthma who are taking the combined pill, or considering it, should speak to their doctor about their contraception options and their asthma symptoms. Whichever contraception they use, it’s vital that women with asthma use their preventer inhalers and any other prescribed treatments regularly.”