Premature babies who receive morphine may grow up to be more sensitive to pain, a study on rats suggests.
Premature babies may require many painful procedures
US researchers found rodents given the drug just after birth later needed higher doses of morphine to kill pain than counterparts in a placebo group.
Morphine is a standard part of care for premature babies, who may need hundreds of painful treatments and tests.
The study reported in New Scientist, raises questions, if not answers, about the practice, experts say.
Researchers at the University of South Carolina treated newborn rats with morphine injections for the first nine days of life and tested their pain responses a few weeks later.
Rats are born so immature that their early development is comparable with that of a premature baby.
After nearly six weeks, broadly equivalent to a human becoming a teenager, the morphine rat appeared to be more sensitive to be pain than the rat given the placebo.
For instance it withdrew it paws more readily when these were heated with a lamp, and when pain was induced appeared to need more morphine to quell it.
About 3% of babies in the US and UK are born so prematurely they need treatments which in turn are believed to require pain relief.
Their pain is judged on their physical response, and the drug is administered accordingly.
This research does raise questions about that practice, but other studies have produced contradictory findings about children and pain.
For instance, a Canadian study found that infants who were circumcised without anaesthetic appeared more sensitive to pain when given their immunisation injections later on that children who had not been circumcised.
"Of course if there are adverse affects of giving morphine to infants then we need to look into them, and this is a very interesting - and indeed plausible - study," says Malcolm Levene, professor of paediatrics at Leeds School of Medicine.
"But ultimately we are interested in the option which causes the least amount of harm. The mortality rate is higher for unsedated babies, not to mention the fact that it would be totally unacceptable to wilfully expose newborn infants to severe pain."