Nursing mothers who use painkillers that contain codeine could be putting their infants at risk of a deadly overdose, Canadian doctors warn.

Codeine is metabolized into morphine and is commonly used for pain relief in combination with acetaminophen.

While the American Academy of Pediatrics lists codeine as compatible with breastfeeding, some women may end up metabolizing it into morphine at a faster rate, potentially exposing a breast-fed infant to an excess dose, according to researchers at the Hospital for Sick Children and the University of Western Ontario.

'Our study confirms that codeine as a treatment for pain may be unsuitable and cannot be considered safe for all breastfed infants.'—Dr. Gideon Koren

"With nearly half of all infants in North America being delivered by caesarean section or after episiotomy, there is clearly a requirement for pain relief for mothers," said Dr. Gideon Koren, director of the Motherisk program at Sick Kids, and lead author of the study published in Wednesday's online issue of the journal Clinical Pharmacology & Therapeutics.

"However, our study confirms that codeine as a treatment for pain may be unsuitable and cannot be considered safe for all breastfed infants."

Koren found 17 of 72 infants studied became sedated or experienced abnormal breathing after their mothers took codeine.

Of the 17 cases showing signs of overdose, only three of the mothers had a known genetic variant that increases their risk of metabolizing codeine more quickly; the other 14 did not.

The findings mean that mothers without the variant may also pass on toxic levels of morphine to their babies, Koren concluded, extending his previous findings on the link.

Mothers of infants showing symptoms had an average 59 per cent higher dose of codeine than those whose infants appeared healthy, the team found.

Watch for sleepiness

Mothers taking painkillers containing codeine should be extremely careful, and watch their infants for increased sleepiness or other signs of overdose, Koren advised. Parents should also watch for signs of overdose in children who are taking codeine.

The advice came too late for Rani Jamieson, who lost her son Tariq less than a month after he was born, in April 2005.

Jamieson told CBC Newsworld on Thursday that was frustrated and angry that Tariq's death was avoidable, and she hopes her tragic case will serve as a caution for other mothers.

One caveat of the research is that Koren's study was based on mothers recalling how their babies responded to the drugs, rather than testing the baby's bloodstreams for codeine and morphine.

The good news is that breastfed babies whose mothers are taken off the drug show dramatic improvements, Koren said.