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Doctors have been put on alert after the death of a baby whose breastfeeding mother was prescribed the pain-killing drug codeine.
People convert codeine into morphine at different rates, meaning some women transfer high levels of the toxic substance to their infants through breast milk.
New Zealand's medical regulation authority Medsafe has advised GPs to "carefully consider the risks and benefits" before prescribing it to breastfeeding mums after the death in Britain of an otherwise healthy 13-day-old baby.
A report in British medical journal Lancet said the infant's mother was an "ultra-rapid metaboliser" of codeine, and her breast milk contained toxic levels of morphine.
Medsafe medical adviser Dr Stewart Jessamine said the death was a "rare event" but New Zealand doctors treating breastfeeding mothers should prescribe the "smallest dose" of codeine for the "shortest period of time". He said babies who became "unusually sleepy" after their mothers took codeine should be taken to the doctor.
Other symptoms include the baby having difficulty breathing or feeding and the mother suffering nausea.
Medsafe advises mothers taking codeine to seek urgent medical attention if they experience any of those symptoms. Doctors cannot tell if a mother has the genetic tendency to process codeine quickly.
One in 10 European women is estimated to be an ultra-rapid metaboliser but there are no figures for Maori or Pacific Islanders.
Breastfeeding Association national co-ordinator Marcia Annandale said the death did not mean codeine was unsafe or that breastfeeding was hazardous.
"This is a tragic death, but codeine has been safely used by millions of breastfeeding mothers. This mother should have been told of the potential side-effects though."
Codeine is combined with other medicines in over-the-counter products such as Nurofen and Panadeine.
Annandale said the dose was low in such medicines and there had been no cases of babies suffering ill-effects after mothers had taken them.
Codeine is rarely prescribed on its own as a painkiller in New Zealand, but it can help ease the after-effects of delivery for some mothers.
Dr Bob Fox, of the Royal New Zealand College of GPs, advised mothers to be cautious with any medicines they took while breastfeeding.
