It is nearly impossible to organise drug tests involving premature babies
Some medicines routinely given to premature babies expose them to potentially harmful levels of chemicals, British research suggests.
Doctors at the University of Leicester looked at liquid medicines given to 38 babies in a special care unit.
Some contained chemicals linked to nerve damage, they wrote in Archives of Disease in Childhood.
However, premature baby charity Bliss said that such medicines could be "vital" in saving the baby's life.
Although prescription medicines need to be rigorously tested before being licensed for use in the UK, this does not necessarily mean they have been tested with children, or babies, in mind.
In particular, it is virtually impossible to organise tests which involve premature babies.
Instead, doctors must often use their judgement to work out the safe dose for a tiny baby of a medicine designed for an adult or a much older child.
All the babies in the study had been born earlier than 30 weeks, and given a range of treatments for between two and nine weeks, including iron and vitamin drops, to drugs such as dexamethasone, designed to aid their underdeveloped lungs.
Because the babies could not be given tablets, all the medicines were given in liquid forms, many of which need additional chemicals in their formulation to allow them to be absorbed properly.
The researchers calculated that the baby's exposure to alcohol in these medicines ranged from 0.2ml to 1.8ml per week - equivalent to between one and seven units of alcohol.
In addition, many of the babies exceeded their equivalent of the maximum adult intake of sorbitol, a mix of dexamethasone and iron, and chemicals such as propylene glycol, which has the potential, they said, to cause nerve damage at higher levels.
The researchers said that there was no alternative in some cases other than to add the chemicals to the medicines, but said that more work was needed to evaluate the risk to these babies.
Dr Hitesh Pandya, who led the research, said: "This study documents a worldwide problem.
"It shows that the collection of medicines given to babies may ultimately lead to them being exposed to harmful chemicals with the potential for short and long-term toxic effects."
However, his colleague Dr Andrew Currie urged parents not to panic. He said: "These chemicals can be found in foods all around the world.
"What the study highlighted is that we have a greater understanding of the side-effects of the drugs than we do of the chemicals that many of these drugs are mixed with; there just simply hasn't been enough research done."
However, Andy Cole, chief executive of Bliss, said that while the research was "very interesting", and might improve outcomes for the most vulnerable babies, the drugs were frequently necessary to keep premature babies alive.
"Many medicines used to treat premature and sick babies are not licensed in the same way as drugs for adults.
"Often these medicines are vital in saving a baby's life."
A spokesman for the Medicines and Healthcare Products Regulatory Agency said that any product containing such chemicals had to justify their use in order to get a licence.
Ingredients such as these also had to be declared on the label, he said, making doctors fully aware that they were included.