They said that substances contained in the medicines are sometimes above the level recommended for adults and could cause nerve damage.
As medicines are rarely tested in children, there is little evidence about their safety for younger people. The scientists at Leicester Royal Infirmary are calling for action to be taken to remedy the situation.
The study followed 38 babies born less than 30 weeks into pregnancy and who weighed no more than 1,500g (3.3lb) each.
They were given a range of treatments, from iron and vitamin drops to furosemide (a diuretic used to lessen fluid retention or treat congestive heart failure or lung disease) and dexamethasone (a steroid).
The babies were found to be regularly exposed to more than 20 different excipients - substances used to ease the administration, absorption or preservation of a medicine or improve its taste and appearance.
‘Preterm infants are commonly exposed to excipients, some of which are potentially toxic,’ the authors wrote. ‘Strategies aimed at reducing excipient load in preterm infants are urgently required.’
The study was published online in the journal Archives of Disease in Childhood.