Children who take multivitamins may be at a greater risk of developing asthma and food allergies, research suggests.
Asthma rates have been increasing
Researchers from the Children's National Medical Center in Washington say the reason for the apparent link is unclear.
They believe vitamins may cause cell changes that increase the odds of an allergic reaction, but say as yet there is no proof this is the case.
The research, based on more than 8,000 children, is published in Pediatrics.
It was based on data from a US Government study that began following mothers and infants in 1991.
The study found an association between early infant multivitamin intake and asthma among black infants and an association between early infant multivitamin intake and food allergies in formula-fed infants.
It also found an increased risk of food allergies among all children given multivitamins at age three.
Writing in the journal, the researchers say: "Early vitamin supplementation is associated with increased risk for asthma in black children and food allergies in exclusively formula-fed children.
"Additional study is warranted to examine which components most strongly contribute to this risk."
Difficult to draw conclusions
Asthma UK agreed that further research was required.
In a statement, it said: "This study contradicts previous studies that have shown that lower vitamin levels are associated with higher rates of asthma and asthma severity.
"It is extremely difficult to draw conclusions from this study alone.
"It is possible that children who had already become wheezy as infants, and who were therefore more likely to be diagnosed with asthma at three years, were more likely than non-wheezy infants to be given multivitamins to improve their health.
"Additionally, it is hard to diagnose asthma in children under the age of five.
"Hence some children with 'asthma' at three years may have suffered with transient wheeze rather than allergic asthma, which may have disappeared as they grew older."
Frankie Phillips, a nutrition scientist at the British Nutrition Foundation, said the findings were "surprising".
She stressed that the study had not proved that multivitamins were directly responsible for an increased risk of developing an allergy.