Nursing mothers who take codeine medication should be aware that, depending on how quickly they metabolize the drug, their breastfed babies may be at increased risk of morphine overdose, Health Canada warns.

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

Some women metabolize morphine at a faster rate, potentially exposing a breastfed infant to higher levels of morphine in breast milk than expected. Reports of side-effects in infants are rare, but in severe cases infants may die, the department said Wednesday.

Health Canada is working with drug manufacturers to change the labelling on prescriptions containing codeine to highlight the risk to mothers who may be "ultra-rapid metabolizers."

The advisory recommended that nursing mothers take precautions to minimize the risk of morphine exposure:

• Check with a physician before taking any products containing codeine.
• Read the ingredient list of all over-the-counter medications, especially cough or pain medications, to see if they contain codeine.
• If treatment with codeine is necessary, use the lowest effective dose for the shortest period of time.
• People who metabolize codeine very rapidly may experience overdose symptoms themselves such as extreme sleepiness, confusion, or shallow breathing.
• While the amount of overdose is generally not considered life-threatening in adults, if you are nursing a child and experience these symptoms, it is important to monitor your child carefully and talk to your doctor.
• Contact a doctor if your breastfed baby is sleepier than usual or has trouble breastfeeding. Seek immediate medical attention if your breastfed baby has difficulty breathing or is limp after you take a product containing codeine.

The chance of being a fast metabolizer varies among ethnic groups, according to Health Canada:

• North African, Ethiopian and Arab populations have the highest estimated prevalence, from 16 to 28 per 100 people.
• The prevalence is estimated to be one per 100 people for those of Chinese, Japanese and Hispanic descent.
• Among blacks, the estimated prevalence is three per 100.
• For Caucasians, the estimated prevalence is one to 10 per 100.

Since there is currently no test available for general use in Canada to identify ultra-fast metabolizers, careful monitoring of mother and child is the best way to spot those potentially at risk, the department said.

In August, the Society of Obstetricians and Gynecologists of Canada and the Canadian Paediatric Society discussed which painkillers are appropriate to use during breastfeeding in light of the latest research.

Also in August, the U.S. Food and Drug Administration warned nursing mothers taking codeine to watch their infants for increased sleepiness or other signs of overdose.