The risk of injury to newborns of mothers who were taken opioids after giving birth, typically following a caesarean section, is not higher than that of moms who were not prescribed opioids.

The study from Canada was published by The BMJ. According to the researchers, these data should reassure medical professionals and parents that there is little chance of harm to these infants.

Patients are frequently given opioids, such as morphine and codeine, soon after surgery to treat pain. The risk that opioids in breast milk could represent to nursing infants is still up for debate, despite the fact that all opioids pass into breast milk at levels that are not anticipated to affect nursing infants.

To address this uncertainty, researchers in Canada set out to examine whether maternal opioid treatment after delivery is associated with an increased risk of adverse outcomes in infants.

They drew on eight years of healthcare data from 865,691 mother-infant pairs discharged from a hospital in Ontario within seven days of delivery from 1 September 2012 to 31 March 2020.

After exclusions were applied, 85,852 mothers filled an opioid prescription within seven days of discharge and 538,815 did not. Each mother who filled an opioid prescription within seven days of discharge was matched to a mother who did not (control group).

Most (81 per cent) of mothers in the matched cohort delivered by caesarean section. Among the mothers prescribed opioids, 42 per cent received oxycodone, 20% codeine, 19% morphine, and 12% hydromorphone, with an average supply of 3 days.

The researchers then followed all infants for 30 days for a range of serious outcomes including readmission to the hospital, emergency department visits, admission to a neonatal intensive care unit, and death from any cause.

After taking account of other potentially influential factors, including the mother's age and medical history, the researchers found that, of the infants admitted to the hospital within 30 days, 2,962 (3.5%) were born to mothers who filled an opioid prescription compared with 3,038 (3.5%) born to mothers who did not.

Infants of mothers who were prescribed an opioid were no more likely to be admitted to the hospital for any reason than infants of mothers who have not prescribed an opioid (absolute risk increase of 0.08 per cent).

Infants of mothers prescribed an opioid were marginally more likely to be taken to an emergency department in the subsequent 30 days (absolute risk increase of 0.41 per cent), but no differences were found for any other serious outcomes in the infants, including breathing problems or admission to a neonatal intensive care unit, and no infant deaths occurred.

This is an observational study, so can't establish cause, and the researchers acknowledge several limitations, such as a lack of information on the extent to which the drugs were taken, use of other non-prescription painkillers, and breastfeeding status.

What's more, they can't rule out the possibility that some other, unmeasured factors may have affected their results.

Nevertheless, they point out that initial breastfeeding rates in Canada are very high (90 per cent), providing greater confidence in their conclusions.

Findings are also consistent with the fact that millions of new mothers are prescribed opioids after delivery each year, yet no convincing reports have been published of serious opioid toxicity in infants associated with breastfeeding.

As such, they conclude: "Findings from this study suggest no association between maternal opioid prescription after delivery and adverse infant outcomes, including death."