Infants who receive antibiotics during the first 6 months of life have a higher risk of developing certain common allergies (such as allergic rhinitis) and asthma by age 7, according to new research. Researchers reviewed the medical reports of 445 children, about half of whom had received antibiotics within 6 months after birth, and followed them for the development of allergies and asthma until the age of 6 or 7. Antibiotic use was associated with a higher relative risk of allergies and asthma by age 7, compared with infants who didn't receive antibiotics. Broad-spectrum antibiotics were associated with a significantly higher relative risk of asthma.

The association between antibiotic use and allergy and asthma was even stronger among children whose mothers had a history of allergy or asthma and those who lived in a home without pets.

Researchers say their findings support the so-called hygiene hypothesis, which links a relatively clean environment early in life to the development of allergies. According to this theory, early exposure to bacteria may stimulate the immune system and induce tolerance. The findings also suggest that early use of antibiotics alters the developing immune system in a way that invites allergic responses.

Researchers reported their findings at the European Respiratory Society's annual conference in Vienna last fall.