FRIDAY, Aug. 13 (HealthDay News) -- Teens from around the world who regularly take acetaminophen, best known as Tylenol, were more than twice as likely to have asthma as teens who never take the over-the-counter pain and fever reducer, new research finds.

Taking acetaminophen was also linked to an increased chance of eczema and rhinoconjunctivitis, or allergic nasal congestion, in adolescents.

Because the study was epidemiological -- meaning researchers asked teens to report after-the-fact how often they took acetaminophen -- researchers said they can't prove that acetaminophen helps cause asthma.

But the study is one of several in recent years that has linked acetaminophen usage during pregnancy or childhood to an increased risk of developing asthma.

"We cannot assume causation, but the association was found in widely different communities, with widely different patterns of illness and lifestyles," said study author Dr. Richard Beasley, a professor of medicine at the Medical Research Institute of New Zealand. "When you put it together with all of the other studies, clearly there is [cause for concern]."

Nearly 323,000 children between the ages of 13 and 14 who were participating in the International Study of Asthma and Allergies in Childhood answered questionnaires about their use of acetaminophen and history of wheezing, nasal congestion and recurrent, itchy rashes.

"Medium" users of acetaminophen were those who had taken the drug at least once during the prior year; "high" users were those who reported taking acetaminophen at least once a month for the past year.

The risk of having asthma was nearly 2.5 times higher among frequent users, and 43 percent higher among medium users than those who never took acetaminophen.

Frequent users of acetaminophen were also more than twice as likely to have rhinoconjunctivitis as kids who never too the drug, while medium users had a 38 percent greater risk. For eczema, frequent users had a 99 percent increased risk, while medium users had a 31 percent increased risk.

The study also found an association with frequency of use and severity of asthma symptoms. Frequent users of acetaminophen were 2.75 times as likely to say their wheezing was so bad it disturbed their sleep and limited their ability to speak, Beasley said.

The study will be published Aug. 13 on the American Thoracic Society's Web site and will later appear in the American Journal of Respiratory and Critical Care Medicine.

Marc Boston, a spokesman for McNeil Consumer HealthCare, which makes Tylenol, said in a statement that "there are no prospective, randomized controlled studies that show a causal link between acetaminophen and asthma."

He also noted that "the authors of this [new] study also published data in the September 2008 edition of The Lancet. In the press release that accompanied that data, they stated that, 'international asthma guidelines recommend that for both children and adults with asthma, [acetaminophen] is the preferred drug to relieve pain or fever.'"

A second study in the American Journal of Respiratory and Critical Care Medicine, conducted among children in Ethiopia, also found a link between acetaminophen use and asthma, but not eczema.

Researchers followed some 1,065 women during pregnancy and for three years after their child was born. Between ages 1 and 3, about 7.7 percent of children had experienced wheezing.

Children who had taken one to three tablets of acetaminophen in the past month were 88 percent more likely to have asthma, while children who had taken four tablets were more than seven times more likely to have asthma.

"These two studies further contribute to what appears to be mounting evidence of an association between acetaminophen and asthma and potentially other allergic diseases," said Matthew Perzanowski, an associate professor of environmental health sciences at Columbia University's Mailman School of Public Health.

So what should parents do?

Experts said a link between ibuprofen, another over-the-counter pain and fever reducer, and asthma has not been reported. Yet experts continue to recommend acetaminophen for asthmatic children, Beasley said, because in some asthmatics, ibuprofen may bring on or worsen asthma symptoms.

"Acetaminophen is still the preferred drug for children to take who have asthma. Its safety profile is better than ibuprofen, and our findings don't change that," Beasley said.

Yet limiting the use of acetaminophen is probably a wise idea, said Dr. Andy Nish, an allergist at the Allergy and Asthma Care Center in Gainesville, Ga.

"My personal opinion is it would be reasonable, particularly in families that had the tendency toward allergic diseases such as asthma, hay fever and eczema, to use acetaminophen judiciously, and perhaps consider the use of an alternative," Nish said.

Young children should not be given aspirin, he noted.

Researchers believe acetaminophen may impact the development of asthma by altering the body's immune response. Previous studies have suggested that acetaminophen may lower levels of glutathione in the lungs, which is involved in detoxifying the body, Perzanowski said. Other research has found that children with asthma have lower levels of glutathione in the lungs than those without asthma.

Asthma rates have been climbing in developed nations over the last 30 years, leading researchers to wonder what about the modern Western world could be contributing. Previous research found using acetaminophen during the first year of life increased the chances of having asthma at age 6 or 7.

Beasley, Perzanowski and other experts said the next step is conducting a randomized, controlled trial -- the gold standard of medical studies -- to see if acetaminophen really is the cause.

But there remain other possible explanations for the asthma-acetaminophen link. Children who have asthma or who will go on to receive an asthma diagnosis may develop more colds and respiratory illnesses, or may seem to have worse colds, than other kids, Perzanowski said. Colds can cause fever, which may prompt parents to give acetaminophen.

Frequency of acetaminophen use varied among countries, with only 2 percent of children in Taiwan taking it more than once a month, while 68 percent of kids in Nigeria did.

More information

The U.S. Centers for Disease Control and Prevention has more on asthma.