Infants who are given the antibiotic amoxicillin are at an increased risk of dental problems later in life, University of Iowa researchers suggest.
Disease can affect the development of tooth enamel cells
They said the drug appears to be linked to enamel damage in permanent teeth.
The more children took the drug, the more teeth were affected, Archives of Pediatrics and Adolescent Medicine study found.
But a UK expert said it was more likely the diseases themselves had caused the damage, rather than the drug.
Amoxicillin is one of the most common antibiotics used among children, used for conditions such as middle ear infections.
Enamel defects can show clinical signs ranging from barely noticeable white flecks, to pits and brown stains.
The researchers followed 579 infants from birth until they were 32 months old, using questionnaires every three to four months to gather information on fluoride intake and amoxicillin use.
By the age of one, three-quarters of the subjects had used amoxicillin.
By 32 months, 91% had used amoxicillin.
It was found that amoxicillin use from three to six months doubled the risk of dental fluorosis.
This is a condition usually linked to exposure to excess levels of fluoride in which the normal maturation of dental enamel is blocked due to disruption of the ameloblasts - the cells which produce this hard protective coating.
Writing in Archives, the team led by Dr Liang Hong, said: "The results of our study show that amoxicillin use during early infancy seems to be linked to dental fluorosis on both permanent first molars and maxillary central incisors."
"Duration of amoxicillin use was related to the number of early-erupting permanent teeth with fluorosis."
The researchers said the association remained even when other factors such as fluoride intake, infections and breastfeeding were taken into account.
Dr Hong and his colleagues say more research is needed into the link.
But they add: "Amoxicillin use in infancy could carry some heretofore undocumented risk to the developing teeth.
"While the results of this one study do not warrant recommendations to cease use of amoxicillin early in life, they do further highlight the need to use antibiotics judiciously, particularly during infancy."
Dr Paula Waterhouse, a dental paediatrician at the University of Newcastle, took issue with the research findings.
She said research had shown that problems with enamel maturation were more likely in children who suffer from infections.
She told the BBC News website: "My feeling is that despite in-depth statistical analyses it is likely to be the illness itself which is causing the developmental defects in enamel.