The research was conducted while Risnes was a visiting researcher at Yale University, and resulted in the recent online publication of the article in the American Journal of Epidemiology.

"Asthma is a very common disease. At the same time, about one-third of infants in our study were treated with antibiotics by the time they were six months old. This proportion is about 30 per cent in other Western countries," says Risnes.

The Yale study followed 1400 children and mothers from the beginning of pregnancy until the children were six years old.

"We found that the risk that children would have asthma as six year olds was 50 per cent higher when they had been given antibiotics as a baby. That is a significant increase," Risnes says.

While previous research has suggested an association between asthma and antibiotics, those studies may have been biased because antibiotics are used to treat respiratory tract infections that could themselves be early symptoms of asthma.

The study sought to eliminate this bias by excluding children who were treated for respiratory infections from the study. The study also considered a long list of other risk factors -- such as whether or not the mother, father or a sibling had asthma. That aspect also brought a surprise, Risnes said.

"We actually found that the relationship between antibiotic use in the first six months of life and asthma was particularly strong in children from families without a history of asthma," said Risnes.

"What we think is that antibiotics interfere with the beneficial bacteria found in the gut. These bacteria aid in helping the baby's immune system to mature. When the bacteria are affected, it can cause the child to have an "immature" immune system, which in turn leads to allergic reactions," says Risnes.

She believes that the results should remind doctors and policymakers of the consequences of overuse of antibiotics. While in Norway, for example, the policy is to limit the prescription of antibiotics "this is an additional reminder to doctors and parents that we should avoid unnecessary use whenever possible," she said.

Note: If no author is given, the source is cited instead.

• Asthma
• Allergy
• Infant's Health
• Infectious Diseases
• Children's Health
• Chronic Illness

• Candidiasis
• Upper respiratory tract infection
• Wheeze
• House dust mite
• Hypoallergenic
• Sex linkage