Babies given antibiotics are more likely to develop asthma and other allergies, research suggests.
Baby's guts may be affected
Scientist at Henry Ford Hospital in Detroit say doctors should be more cautious about prescribing the drugs to babies under six months.
They believe the drugs may interfere with the development of the gut and therefore the immune system.
It is known that antibiotics can kill off beneficial bacteria which live in the gut.
More than a million UK children have been diagnosed with asthma, and allergy rates in general are rising.
Researcher Christine Cole Johnson said: "I'm not suggesting that children shouldn't receive antibiotics, but I believe we need to be more prudent in prescribing them for children at such an early age."
Dr Johnson's team studied 448 children, whose development was tracked for the first seven years of their life.
By the age of seven, children given at least one antibiotic during their first six months were 2.5 times more likely to have developed asthma than those who were not given the drugs at such a young age.
Overall, they were 1.5 times more likely to have developed some form of allergy.
Children whose mothers had a history of allergies were particularly at risk. Early use of antibiotics in this group doubled their risk of developing an allergy.
Babies who were breastfed for more than four months, and who received antibiotics in their first six months were three times more likely to develop allergies - although they were no more likely to develop asthma.
Exposure to pets, however, seemed to have a protective effect. Those given antibiotics who lived in a family with fewer than two pets had 1.7 times the risk of allergies and three times the risk of asthma.
However, when a family had two or more pets, the risk was normal.
The biggest risk of all - an 11-fold increase - was found among children who were prescribed a broad-spectrum antibiotic, such as penicillin, were breastfed for four months, and did not have any family pets.
The researchers also found evidence that the more courses of antibiotics a child received during their first six months, the higher was their risk of developing an allergy.
A spokesman for the National Asthma Campaign said research into the area had proved inconclusive.
"Whilst this is an interesting study, further research is needed to understand the complexities of the relationship between these factors."
Details of the research were presented at the European Respiratory Society's annual conference in Vienna.