Oral contraceptives and the risk of asthma attacks: a population-based cohort study.

Background: The role that sex hormones play in asthma remains unclear. The oral contraceptive pill (OCP), commonly used by younger women, acutely increases sex hormones providing an opportunity to observe their effect. The objective of the present study was to evaluate the association between OCP and asthma attacks. Methods: Using the UK's Clinical Practice Research Datalink, linked to hospital admission and mortality data, 2004 to 2020, we observed women with asthma (18-50 years), comparing OCP never-users to new-users; separated into a combined oral contraceptive (COC) cohort and progestogen-only pill (POP) cohort. We applied inverse-probability of treatment weighting and Cox proportional hazards, accounting for demographics, asthma severity/control and comorbidities. Additionally, we stratified by potential modifiers: age, body mass index (BMI), blood eosinophils (normal <0.3×10 Results: 132 676 and 129 151 women were eligible for the COC and POP cohorts, respectively. There was no association between COC, or POP, and asthma attacks (COC: weighted-HR 1.00, 95% CI 0.89-1.13; POP 1.11, 95% CI 0.97-1.28). However, POP association was modified by asthma phenotype and corticosteroid use, but not BMI, after accounting for asthma severity/control, demographics and comorbidities. In the POP users, women who were younger than 35 years (weighted-HR 1.39, 95% CI 1.12-1.72), those with eosinophilia (weighted-HR 1.24, 95% CI 0.97-1.58) or those with lower corticosteroid use (weighted-HR 1.20, 95% CI 1.03-1.40) had an elevated risk of asthma attacks. Conclusions: Commencing exogenous progesterone without an oestrogen component (POP) was associated with increased asthma attacks in asthma phenotypes including in younger women, eosinophilic asthma and women with lower corticosteroid use.