An epidemiological study suggests that use of oestrogen-only hormone replacement therapy (HRT) increase the risk of newly-diagnosed asthma during and shortly after use, although the absolute risk is small.

Evidence suggests a link between oestrogen levels and asthma, and some studies have found an association between exogenous oestrogen use and asthma diagnosis. Most previous studies have been cross-sectional, however, and this study aimed to determine whether use of HRT was associated with an increased risk using a well established cohort with long-term follow-up. The authors used data from an existing cohort of French women, the E3N cohort, which started in 1990. Participants were born between 1925 and 1950 and completed questionnaires on medical history and lifestyle every two years. Incident cases of asthma were women who indicated that they were not asthmatic at baseline and subsequently met the American Thoracic Society definition of asthma. HRT use was obtained from the questionnaires and classified into type of oestrogen use and type of progestogen use (including none). Follow-up was from entry into the cohort, to July 2002 or first asthma attack.

The original cohort contained 98,995 women. After exclusions, most often due to lack of data on asthma status, there were 57,664 for analysis with 495,448 person-years of follow-up; 41,526 (72%) reported HRT use at some point. There were 569 cases of incident asthma during the follow-up period (1.15/1,000 per year). Overall, women who had recently taken HRT had a non-significantly higher risk of asthma compare to never-users (hazard ratio [HR] 1.20; 95% CI 0.98 to 1.46). When the analysis was restricted to recent oestrogen-only HRT users, the increased risk was statistically significant (HR=1.54, 95% CI 1.13 to 2.09).

The authors conclude that use of oestrogen-only HRT was associated with an increase in the risk of newly-diagnosed asthma in this study cohort. The effect was found only in recent users and those who had stopped within the previous 1.5 years. They discuss the potential strengths and limitations of their study, and comment that the increased risk should be judged in relation to the benefits and other risks of HRT.