Anaesthesia during a short surgery doesn't harm a baby's brain development, according to an experiment involving hundreds of infants in seven countries.
While the study can't answer broader safety questions about repeated or prolonged anaesthesia, it may ease the worries of millions of parents whose children have been put to sleep for common procedures.
"These findings should be reassuring," said Dr Mary Ellen McCann of Boston Children's Hospital. An hour of surgery with general anaesthesia "is safe for your child in early infancy". She helped lead the study published on Thursday in the medical journal Lancet.
It involved 447 babies needing hernia repairs. The babies, mostly boys, were randomly assigned to get either anaesthesia with gas, or an injection that blocks sensation below the waist.
Since both techniques are commonly used, it was ethical for the researchers to set up an experiment. They found no evidence of harm to brain development when they tested the children at age 2.
Finally, at age 5, the children took IQ tests and both groups' average scores were in the normal range. There were no differences in parent-reported problems such as autism, attention deficit disorder or speech delays.
"The level of evidence is strong," said Dr Santhanam Suresh of Lurie Children's Hospital in Chicago, who wasn't involved in the research. The findings mean doctors "should not shy away from using general anaesthesia in children undergoing simple pediatric procedures".
Since 84% of the babies in the study were boys, it's unclear how the results apply to girls.
In the study, the anaesthesia lasted less than an hour on average. Longer exposure could be more dangerous, as could anaesthesia for multiple surgeries, McCann said, so it's unlikely the Food and Drug Administration will change existing warning labels on anaesthesia drugs for children.
Uncertainty about the drugs stems from studies showing brain damage in baby animals. Figuring out how these drugs affect children has been difficult, though, because very sick kids who get the most anaesthesia also have other problems that can cause trouble with learning. That makes it tough for scientists to sort out what causes problems.
Funding came from government and scientific groups in the United States, Australia, Canada, New Zealand, Italy, the United Kingdom and the Netherlands.
Next, other researchers will study a new combination of anaesthetic drugs in another randomised experiment , but those results won't be known for several years.