The suspicion stems in part from its chemical structure, which resembles that of theophylline, a common asthma medication that relaxes the airway muscles and relieves wheezing, shortness of breath and other respiratory problems. Indeed, when caffeine is ingested and broken down by the liver, one byproduct is small amounts of theophylline.
In a 2007 study in the Cochrane Database of Systematic Reviews, researchers pooled and analyzed the results of a half dozen clinical trials looking at the effects of caffeine on asthmatics. They found that caffeine produced small improvements in airway function for up to four hours, compared with a placebo, and that even a small dose — less than the amount in a cup of Starbucks coffee — could improve lung function for up to two hours.
In other words, in a pinch, a cup of coffee or strong tea might provide some momentary relief.
But the improvements are very slight, studies show …