A paper in the

) described an increase in infantile hypertrophic pyloric stenosis in babies given prophylactic erythromycin after exposure to whooping cough. Now more data have been reported from a hospital in Indianapolis.

The retrospective cohort study (

) included 14 876 infants born between June 1993 and December 1999. Forty-three (0.3%) of these infants developed pyloric stenosis, most of them (29) between 3 and 6 weeks of age. Thirty-six of the infants with pyloric stenosis were boys but erythromycin had been prescribed equally for boys and girls (3.2% and 3.1%). No infant given erythromycin after 2 weeks of age developed pyloric stenosis but 6 of 226 (2.7%) prescribed erythromycin in the first 2 weeks did so. Five of 182 (2.7%) infants prescribed erythromycin in the first week and one of 44 (2.3%) prescribed erythromycin in the second week developed pyloric stenosis (relative risk 10.5 for erythromycin in the first 2 weeks). All of the infants who developed pyloric stenosis had been given erythromycin for 14 days or longer. None of 1243 infants treated with erythromycin eye ointment developed pyloric stenosis. There was a suggestion (not statistically significant) of an increased risk of pyloric stenosis after treatment of the mother with a macrolide antibiotic in the last 10 weeks of pregnancy, especially if the infant was also treated.

Erythromycin given for 14 days or longer to infants up to 2 weeks old increases the risk of pyloric stenosis tenfold.