The U.S. Food and Drug Administration is warning that nursing infants may be at risk of morphine overdose if their mothers take codeine and have a genetic mutation that makes them ultra-rapid metabolizers of the painkiller.

The agency issued the warning Friday after reviewing information on the subject, including a medical journal report about the death of a 13-day old breastfed infant who died from morphine overdose.

The morphine levels in the mother's milk were found to be abnormally high after taking small doses of codeine prescribed to treat pain for an episiotomy (a procedure to enlarge the vaginal opening before childbirth.).

A genetic test showed that the mother was an ultra-rapid metabolizer of codeine, meaning that her body metabolized codeine into morphine much faster and more completely than others. Mothers who are ultra-rapid metabolizers may have higher-than-usual levels of morphine in breast milk, risking a morphine overdose in their babies.

"Our best advice to physicians prescribing codeine-containing products to nursing mothers is to prescribe the lowest dose needed for the shortest amount of time," said Dr. Sandra Kweder, deputy director of the Office of New Drugs in FDA's Center for Drug Evaluation and Research.

"And nursing mothers should always consult their physicians before taking any codeine containing products."

Codeine has long been considered the safest choice among narcotic pain relievers for nursing women and their babies. Dr. Gideon Koren, director of the Hospital for Sick Children's Motherisk Program, told CTV's Canada AM last year that about 150,000 mothers a year receive codeine following childbirth, often to alleviate the pain of episiotomies and caesarean sections.

"Codeine is assumed by the authorities to be safe during breastfeeding," Koren noted. "But if you have this genetic change, you will be overdosing."

Around one per cent of Caucasians are estimated to have multiple copies of the "ultra rapid metabolizer" gene. And almost 30 per cent of Ethiopians and Saudi Arabians and 10 per cent of southern Europeans are estimated to have the gene in multiple copies.

Since many doctors are not aware of the risks of breastfeeding for ultra-rapid codeine metabolizers, the FDA is requiring manufacturers of prescription codeine medicines to include information about codeine ultra-rapid metabolism in drug package insert information.

In addition, FDA has posted information about this issue on its website for patients.

Nursing mothers taking codeine (or other narcotic pain relievers) should know how to watch for signs of overdose in their babies. Signs of morphine overdose include increased sleepiness, difficulty breastfeeding, breathing difficulties or limpness.

Breastfed babies normally nurse every two to three hours and should not sleep for more than four hours at a time.

In the case of the 13-day-old infant who died, the first-time mother became concerned about her baby's lethargy and brought the baby to her doctor when the baby was seven days old. The doctor did not recognize the signs of morphine overdose and advised her to continue breastfeeding.

By day 12, the infant had grey skin; on day 13, he died at home.