US Food and Drug Administration warns about codeine side-effects when prescribed to nursing mothers. The FDA issued the warning and gave more information on the subject after a medical journal reported the death of a 13-day breastfed infant due to morphine overdose.

“When codeine enters the body and metabolizes, it changes to morphine,” explained Dr. Sandra L. Kweder, the FDA’s deputy director of the Office of New Drugs. The morphine level in the mother’s milk was higher than normal, after taking a small dose of codeine to relieve episiotomy pain. A subsequent genetic test proved that the mother was an ultra-rapid metabolizer of codeine. Ultra-rapid metabolizers have a biologic variation in a liver enzyme that makes them transform codeine into morphine more quickly than others.

Codeine is prescribed often to women that have recently given birth after caesarean sections and episiotomies, but it is contained by low-grade painkillers and other non-prescription cough medicine.

Far from trying to discourage breastfeeding, the FDA recommends that “physicians prescribing codeine-containing products to nursing mothers should prescribe the lowest dose needed for the shortest amount of time,” Kweder said. “And nursing mothers should always consult their physicians before taking any codeine containing products.”

The codeine has been the safest narcotic painkiller for nursing mothers and their babies, but FDA considers that it is essential that manufacturers of codeine-based medicines include information about ultra-rapid metabolism on drug package.

Also, nursing mothers should watch for signs of morphine overdose in their babies, after taking codeine. Breasted babies usually nurse every two or three hours and don’t normally sleep more than four hours at a time. Increased sleepiness, breathing difficulties, difficulty breastfeeding and limpness are signs that should alert the mother and take the baby to an emergency room.

The odds of being an ultra-rapid metabolizer vary from 1 per 100 people to 28 per 100 people, depending on the population. There is an FDA cleared test for determining a patient's CYP2D6 genotype. The test is not routinely used in clinical practice but is available through a number of different laboratories.