Babies who receive "one brief dose" of general anaesthesia are unlikely to suffer brain damage at the age of five - but safety to longer exposures remains unclear.
Auckland's paediatric anaesthetist Dr Niall Wilton said concern has been growing over the past 30 years that the use of general anaesthesia in babies may not be as innocent as previously thought.
In order to test this theory, which stemmed from multiple animal studies, Wilton has been involved with a groundbreaking worldwide study tracking the mental capacity of 360 children who underwent hernia repair surgery as babies.
They were compared to another 360 children who had not been exposed to general anaesthesia.
"The study showed absolutely no difference between the groups," Wilton told the Herald.
Children were given a psychological and IQ test at the age of two and then again when they turned five, he said.
"For these children who had been exposed to less than an hour of general anaesthesia there was no increased problem in the primary outcome which was intelligence, or the behavioural development."
An IQ test at the age of five was a good indicator of how people behave later in life, Wilton said.
"So if you do ok in the 5-year-old test then the chances are you will do ok when you're older. Likewise, if you are having behavioural problems at the age of five then that is likely to progress later on," he said.
The study involved researchers and doctors from 28 hospitals across Australia, New Zealand, the US, Canada and Europe tracked babies who had the surgery in February 2007 until they turned five in January 2013.
Twenty-five babies who had undergone hernia repair surgery at Auckland's Starship Hospital took part in the study.
Wilton said these latest findings are the strongest evidence to date that one hour long exposures to general anaesthesia is safe in young children.
However, he said it was still unknown the impact of general anaesthesia on children who had been exposed to the drug for longer than one hour.
A Kiwi story:
Jai Costello has undergone nearly 100 surgeries yet his mum says he's top of his class.
The 9-year-old Tauranga boy suffers from laryngeal papilloma, a growth on his airway.
It was discovered after Jai was rushed to Auckland's Starship Hospital at aged 20 months with breathing problems that had worsened since birth.
"His airway was so blocked he was breathing through [a gap] the size of a needlepoint. It's pretty incredible," his mum Kayla Mackenzie said.
If the growth hadn't been cut back, within a few weeks Jai would have stopped breathing, Mackenzie said.
He now has surgery every month to six weeks to trim the growth.
But despite it all, Mackenzie says he is "thriving at school."
"His school report is always amazing. He's top of the class at maths and does a couple of years above.
"I don't think his surgeries have impacted his learning or behavioural development at all. In fact he hates when we pull him out of school," Mackenzie said.
She describes her "resilient boy" as having the ability to "just get on with it."