Asthma and Tylenol: How strong is the evidence?

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Yet another new study — this one is in the American Journal of Respiratory and Critical Care Medicine — is showing a link between asthma and acetaminophen, the active ingredient in Tylenol. Researchers have found that, among 320,000 kids in 50 countries, 13- and 14-year-olds who take acetaminophen are more than twice as likely to have asthma as teens the same age who never take the common over-the-counter painkiller. Other trials have found a similar link in both kids and adults. One even found a link between mothers’ acetaminophen use during pregnancy and later asthma diagnosis in her children.

But is acetaminophen really causing the asthma? That question is much, much harder to answer. Almost all of the asthma-acetaminophen studies to date, including the one released today, have been what’s called “cross-sectional”: They study whether people at a given point in time use acetaminophen, and also whether those people have asthma. But there’s no way to separate cause and effect in that kind of study design. It is possible that acetaminophen causes asthma. It’s also possible that asthma symptoms lead people to take more painkillers, or even that something else entirely (like general poor health) makes people both more susceptible to asthma and more likely to take acetaminophen. Unfortunately, none of the asthma-acetaminophen studies to date have been randomized controlled trials — the gold standard of research design. That kind of trial tends to be much more expensive than the cross-sectional variety.

So the link between acetaminophen and asthma is clear. Acetaminophen users do have more asthma. The reason for that link, however, is not clear. More more research is needed.

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