MEDIA BRIEFING
13th March 2008
Aspirin reduces
the risk of developing asthma in women
Summary
Women who regularly take low dose aspirin have a lower risk of
developing asthma, a new analysis of the Women's Health Study
suggests.(1)
The study showed that, over a 10-year period, women who took aspirin
100 mg every other day had a 10 per cent lower risk of developing
asthma compared with women taking placebo. This effect was the
same regardless of age (over 45), smoking status and exercise
levels but was not apparent in women who were obese.
This finding follows earlier evidence that aspirin is associated
with a reduced risk of developing asthma in healthy men (in the
Physicians' Health Study (2)) and in women (in the Nurses' Health
Study (3)).
How was the new study carried out?
The Women's Health Study was a randomised, placebo-controlled,
double-blind trial to evaluate the risks and benefits of taking
aspirin (100 mg every other day) and/or vitamin E (600 IU every
other day) in 39,876 American women. The trial began in 1993 and
ended in 2004
Women who joined the study were health professionals aged at least
45 years. They were apparently healthy; women who already had
asthma were excluded from this analysis, leaving a study population
of 37,270.
What were the results?
This analysis compared the incidence of newly diagnosed asthma
in women who were taking aspirin with those who were not.
During an average follow-up period of 9.7 years, a total of 1,835
new cases of asthma were identified. Of these, 872 occurred among
women taking aspirin (4.68 per cent) and 963 among those taking
placebo (5.17 per cent). This is equivalent to a 10 per cent reduction
in the relative risk of asthma in women taking aspirin.(a)
The risk reduction among women taking aspirin became apparent
after 6 years and was increasing continuously (that is to say,
longer aspirin use was associated with a lower risk of asthma).
The risk reduction associated with aspirin was not affected by
age, smoking status, levels of exercise, use of hormone replacement
therapy, or vitamin E use. However, there was no risk reduction
among women who were obese.(b)
Interpretation
The authors state:
In this large, randomised, double-blind, placebo-controlled trial
in apparently healthy women, 100 mg aspirin every other day significantly
reduced the relative risk of newly reported adult onset asthma
diagnosis by 10 per cent. The magnitude of the reduction was similar
across subgroups of age, smoking status, exercise, postmenopausal
hormone intake and randomised vitamin E intake but was absent
in those who were obese.
They conclude:
However, before public recommendations are provided, results from
randomised trials are needed that are specifically designed to
test whether low-dose aspirin reduces the risk of asthma.
They note that this is not the first study to find an association
between taking aspirin and a reduced risk of asthma.
• The Physicians' Health Study(2) found that men who took
aspirin 325 mg on alternate days had a 22 per cent lower risk
of developing asthma compared with those taking placebo.
• The Nurses Health Study,(3) an observational study comparing
women who reported taking aspirin on at least 15 days a month
with those reporting no use, found that aspirin was associated
with a 40 per cent lower risk of developing asthma.
The authors caution that aspirin may worsen symptoms in a minority
of people who already have asthma.
How might aspirin affect the risk of developing asthma?
The mechanism by which aspirin reduces the risk of developing
asthma is unknown. It is possible that aspirin alters the immunological
events that affect the airways in people with asthma via its effects
on the enzyme cyclo-oxygenase. Asthma is associated with a particular
type of inflammatory reaction known as the Th2 response (due to
the predominance of events associated with Th2 lymphocytes); aspirin
may inhibit this and instead promote a Th1 response, reducing
susceptibility to asthma. The authors say this theory is biologically
plausible but there is currently no definitive evidence to support
it and alternative mechanisms may also be important.
It is also unknown why aspirin appeared to have no effect in obese
women. It may be because asthma is different in this group or
because the dose of aspirin was proportionately smaller when expressed
according to bodyweight.
What were the strengths and weaknesses of the trial?
The strengths of this trial were:
• its design (randomised, placebo-controlled, double-blind)
• the large number of women who participated
• the large number of cases of asthma identified
Its limitations were:
• the participants were apparently healthy, predominantly
white women who were health professionals and it is uncertain
whether the results can be extrapolated to other groups
• it was not specifically designed to test the effects of
aspirin on the risk of asthma (though it had adequate statistical
power to do so)
• asthma diagnoses were reported by the participants (though
this would be random and not expected to favour aspirin)
References
1. Kurth T, Barr RG, Gaziano JM, Buring JE. Randomised aspirin
assignment and risk of adult-onset asthma in the Women's Health
Study. Thorax 2008 doi:10.1136/thx.2007.091447
2. Barr RG, Kurth T, Stampfer MJ et al. Aspirin and decreased
adult-onset asthma: randomized comparisons from the physicians’
health study. Am J Respir Crit Care Med 2007;175:120–5.
3. Barr RG, Wentowski CC, Curhan GC et al. Prospective study of
acetaminophen use and newly diagnosed asthma among women. Am J
Respir Crit Care Med 2004;169:836–41.
Notes
a. The relative risk reduction was calculated as a hazard ratio
of 0.90 with 95% confidence intervals of 0.82 to 0.99. This was
statistically significant (p=0.027).
b. Obesity was defined as body mass index =30 kg/m2, which is
the standard used in the UK.
