Experts caution and advise against using it in young children whenever possible because it may increase the likelihood of major, long-term problems with allergy and/or asthma.

A new study shows that early exposure to antibiotics destroys good bacteria in the digestive tract and can lead to asthma and allergies.

The study, published in Mucosal Immunology, provides the strongest evidence yet that the long-observed link between early childhood antibiotic exposure and later development of asthma and allergies is causal.

“The practical implication is simple,” adds Martin Blaser, senior author, “Avoid antibiotic use in young children whenever you can because it may elevate the risk of significant, long-term problems with allergy and/or asthma.”

In the study, the researchers from Rutgers, New York University, and the University of Zurich said that antibiotics, which are “among the most used medications in children,” change the communities and functions of the microbiome in the gut. These changes in the structure of microbiota can have an effect on host immunity.

Mice that were five days old were given water, azithromycin, or amoxicillin throughout the first phase of the trial. After the mice reached adulthood, scientists exposed them to a typical allergen made by house dust mites. Mice that had received either antibiotic, particularly azithromycin, displayed increased immunological reactions, or allergies.

The second and third stages of the experiment looked into the notion that some beneficial gut bacteria that are necessary for optimal immune system development are killed by early exposure to antibiotics (but not later exposure), which results in allergies and asthma.

Tim Borbet, the study’s lead author, first gave feces samples full of bacteria from the first group of mice to a second group of adult mice that had never been exposed to bacteria or germs before. Some got samples from mice that had been given azithromycin or amoxicillin when they were young. Others got water-fed mouse samples.

Mice that got samples that had been changed by antibiotics were no more likely than other mice to get sick from house dust mites. This is similar to how people who get antibiotics as adults aren’t more likely to get asthma or allergies than those who don’t.

However, things were different for the following generation. Offspring of mice given antibiotic-altered samples reacted more to home dust mites than those whose parents received unaltered samples, just as mice given antibiotics as newborns reacted more to the allergen than those given water.

“This was a carefully controlled experiment,” adds Blaser. 

“The only variable in the first part,” according to the author, “was antibiotic exposure. The only variable in the second two parts was whether the mixture of gut bacteria had been affected by antibiotics. Everything else about the mice was identical.

“These experiments provide strong evidence,” according to the author, “that antibiotics cause unwanted immune responses to develop via their effect on gut bacteria, but only if gut bacteria are altered in early childhood.”

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