It is used far less frequently these days - because of the emerging evidence about long term risks

Professor Henry Halliday, Queen's University Belfast

While babies born prematurely are already at some risk of neurological impairment, this study adds to evidence that dexamethasone may have added to the problem in some cases.

Most UK neonatal units have stopped using the drug in all but the most extreme cases.

Experts say that, while it has undoubtedly saved many lives, there are now too many question marks over its long term safety.

The study was published in the American Journal of Physiology, and was carried out by a team at the University of Michigan.

A group of rats was given a short course of diminishing doses of the drug - equivalent to the doses given to a premature baby with breathing problems.

Stress

They found, in common with other studies, that skeletal growth and brain weight were decreased in the treated animals compared with untreated controls.

While there was evidence of delayed neurological delay in the first two weeks of life, the rats had caught up by the end of the third week.

However, once the rats reached adolescence they were less active than untreated rats in both the light and the dark.

The rats' body chemicals were measured - and a key chemical normally released in response to stress was found at much lower levels in the rats.

This is evidence, say the researchers, that the drug "permanently alters" the function of the central nervous system, which may lead to increased risk for learning impairment.

Extreme cases

A UK expert, Professor Henry Halliday, from the Royal Maternity Hospital and Queen's University Belfast, said that the drug had been valuable in helping vulnerable babies to breathe without the help of a ventilator.

He said: "It is used far less frequently these days - because of the emerging evidence about long term risks.

"We tend to use it only in very extreme situations, in which we believe that the infant will almost certainly die if we do not, and we inform the parents fully about the risks involved.

"There are other ways of getting premature babies off ventilators."