A meta-analysis published in 2009 calculated that children who had taken acetaminophen in the past year had nearly double the risk of wheezing compared with those who had not taken the drug. “We know that acetaminophen can cause increased bronchial constriction and wheezing,” said Mahyar Etminan, a pharmacoepidemiologist at the University of British Columbia and lead author of the study.

Still, Dr. Etminan believes it is not yet clear that acetaminophen itself is responsible for the increasing prevalence of asthma. “Children who take acetaminophen are usually getting it for fever control, and they get fevers because they have viral infections, which on their own are associated with developing asthma later in life,” Dr. Etminan said. “It’s hard to tease out whether it’s the drug or the viral infection.”

Another potential problem, Mr. Etminan said, is that many of the studies required parents to accurately recall how much acetaminophen they gave their children, and how often. Parents whose children have asthma are likely to scrutinize the events that preceded an attack, he said, and thus may be more likely than other parents to recall giving their children the drug.

So far, only one randomized controlled trial has investigated the link. Researchers at Boston University School of Medicine randomly assigned 1,879 children with asthma to take either acetaminophen or ibuprofen if they developed a fever. The results, published in 2002, showed that children who took acetaminophen to treat a fever were more than twice as likely to seek a doctor’s care later for asthma symptoms as those who took ibuprofen.

Other trials are in the works. Dr. Richard Beasley, a professor of medicine at the Medical Research Institute of New Zealand, is just completing a 12-week randomized controlled trial of acetaminophen to see if the drug provokes or worsens asthma in adults. The results of that trial will be completed next year. Dr. Beasley said the highest priority now should be rigorous trials to test whether acetaminophen use in infancy increases the risk of developing asthma.

“I cannot say with 100 percent certainty that acetaminophen makes asthma worse, but I can say that if I had a child with asthma, I would give him or her ibuprofen for the time being,” Dr. McBride said. “I think the burden of proof is now to show that it’s safe.”