In a study published in Nature Communications, scientists document the possible long-term effects of antibiotics when they’re used early in life. Their study involved mice, but the team used the drugs in doses and treatment regimens that mimic those frequently administered in young children.

Dr. Martin Blaser, professor of medicine and microbiology at New York University Langone Medical Center, and his colleagues tested three different antibiotic regimens: one involving amoxicillin, another involving macrolides and a final one that combined the two. They compared these animals to mice that received a placebo. The mice got antibiotics 10 to 15 days after birth, then again 27 days later and finally after day 39. They lived for 160 days, at which point they were sacrificed and their gut bacteria were studied.

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Compared to the mice taking the placebo, the antibiotic-treated animals had less diverse communities of bacteria, and the proportions of the bugs living in their guts were also different. The macrolides seemed to have the biggest effect on reducing microbial richness, while amoxicillin led to abnormally large bones. The changes in the microbiome persisted even to the animals’ death, nearly four months after their last antibiotic dose.

“There are really long-term, probably permanent effects on the microbiome from antibiotics,” says Blaser. “We showed changes in the richness and the community structure, and also the genes present in the bacteria.”

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What this means for humans still isn’t clear from this study, but the findings do provide hints. Other studies that have analyzed the potential effects of antibiotics found that children receiving more rounds of the drugs because of early infections tend to be heavier and are more likely to be obese as adolescents and adults. And the earlier children are exposed to the drugs, the more likely their metabolism is to be affected.

Blaser notes that antibiotics are a necessary and potentially life-saving treatment for some, but for many infections, their risks might be greater than their benefits. “If what we found in mice is true for human children, then this is yet another reason to be cautious in using antibiotics,” he says. “We know there are kids who are severely ill who must have antibiotics. But there is a larger number of kids who are only mildly ill. The question is, what proportion of them really need antibiotics?” Based on the animal data, he says, the first two to three years of life are particularly important for development, and doctors and parents should be judicious about prescribing antibiotics during this sensitive time.

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