Thursday March 13 2008
Could aspirin prevent asthma?
“How aspirin could help women fight asthma” reads the headline in the Daily Mail. Researchers have shown that “older women taking aspirin regularly developed 10% fewer new cases than expected”, the news report underneath says.
This and other reports are based on the results of a re-examination of data from a large US study originally set up to see if a low-dose of aspirin (100 mg) every other day could prevent heart attacks and cancer. The researchers used the data from this study to look at the rates of new diagnoses of asthma in people taking aspirin compared with those taking placebo.
The headline is not an accurate interpretation of the conclusions from this study and does not mean that women with asthma should start taking aspirin. The 10% fewer new cases quoted by the paper and the researchers are presented in a way that hides the absolute, or real, differences between the groups. Only 1,835 of the 37,270 women analysed (4.9%) developed asthma over an average of 10 years and the real difference in the rates of asthma among the women who received aspirin (4.7%) compared with those who received placebo (5.2%) was about 0.5%. This small difference may have arisen by chance, or might be due to unknown differences between the groups.
As the researchers emphasise, there is no known way in which aspirin might affect asthma and there are important side effects to taking aspirin, including stomach pain and bleeding. Some people are also allergic to aspirin. Further studies, designed from the outset to test whether aspirin can prevent asthma, will be required before low doses of a potentially harmful drug can be recommended to be used in this way.
Where did the story come from?
Dr Tobias Kurth from the Brigham and Women’s Hospital in Massachusetts and other colleagues from around the US carried out this research. The Women’s Health Study was supported by grants from the National Heart, Lung and Blood Institute and the National Cancer Institute. It was published in Thorax, a peer-reviewed medical journal.
What kind of scientific study was this?
This was a secondary analysis of data from a randomised controlled trial. The researchers looked at the number of women who self-reported an asthma diagnosis on questionnaires sent out every year as part of the US Women’s Health Study.
The Women’s Health Study was set up to investigate whether a low-dose of aspirin (100 mg) every other day could prevent heart attacks and cancer. Of the 450,000 women who answered an initial screening questionnaire, about 65,000 were willing to participate and were considered eligible for the study. The women were aged over 45 with no previous history of heart disease, stroke, major cancer or long-term illness, including asthma. They had no history of side effects to any study medications. They were not taking aspirin, steroids or anti-clotting drugs, or individual supplements of vitamin A, E or beta-carotene more than once a week. About 25,000 women were excluded in the first three-month “run-in period” after the participants were tested with dummy pills to see if they would be able to take the tablets routinely in the long-term. Overall, almost 40,000 female health professionals were randomised to take part in the study between 1993 and 1996.
What were the results of the study?
During 10 years of follow-up, there were 872 new cases of asthma in the aspirin group and 963 in the placebo group. The difference in the chance of developing asthma over time between the groups was significant. There was a 95% probability that a 10% difference did not occur by chance. There was a hazard ratio of 0.90 (95% confidence interval: 0.82 to 0.99; p=0.027).
What interpretations did the researchers draw from these results?
The researchers conclude that in their “large randomised clinical trial of apparently healthy adult women, administration of 100 mg aspirin on alternate days reduced the relative risk of a newly reported diagnosis of asthma.”
What does the NHS Knowledge Service make of this study?
The original randomised trial collected a large amount of data. However, its aim was to examine the effects of aspirin on heart attacks and cancer, not new onset asthma. The researchers also mention other limitations to their study:
- The study was not designed to test the theory that low-dose aspirin reduces the onset of asthma in women over 45 years.
- The diagnosis of asthma was self-reported, and no objective measures of asthma, such as a measures of lung function, were used. Also, the researchers did not test whether some cases of asthma were being confused with other long-term lung conditions, such as chronic bronchitis, that can occur at similar ages.
- The participants were predominantly healthy, white female health professionals who may not be representative of other groups. The results should not, therefore, necessarily be applied to men or younger people.
The researchers have found a small and statistically significant effect. There is a possibility that this effect could have arisen by chance, but the researchers say that it is in line with findings from other studies. However, further trials specifically designed to test whether low-dose aspirin reduces the risk of asthma are needed before any recommendation to take aspirin to prevent asthma can be made.