(HealthDay News) — New research supports previous findings that erythromycin can increase the risk of infantile hypertrophic pyloric stenosis (IHPS). The research also indicates that azithromycin is associated with a higher risk of IHPS when given to infants under 6 weeks old. The findings were published online February 16 in the Pediatrics.

Matthew D. Eberly, MD, from the Uniformed Services University of the Health Sciences in Bethesda, MD, and colleagues analyzed the records of 1,074,236 children and compared those who received erythromycin or azithromycin in their first three months to those who didn’t. Overall, 2,466 infants developed IHPS and 6,777 infants had received one of the two antibiotics studied. The overall risk of IHPS was small – occurring in 2.29 babies per 1,000.

The researchers found that the use of erythromycin was associated with a 13.3-fold increased risk of IHPS in the first two weeks of life, and a 4.1-fold higher risk when used between the ages of 2–6 weeks. The team also found that azithromycin was associated with an 8.26-fold higher risk when used during the first two weeks of life, and a 2.98-fold higher risk when used in infants between the ages of 2–6 weeks.

“Ingestion of oral azithromycin and erythromycin places young infants at increased risk of developing IHPS,” the authors write. “This association is strongest if the exposure occurred in the first two weeks of life, but persists to a lesser degree in children between 3–6 weeks of age.”

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