In 2005, a mother who had just given birth received codeine to
dull the pain associated with childbirth. In her body, the
codeine was converted into morphine by a drug-metabolizing
enzyme called CYP2D6. Although people carry many variations in
the gene for this enzyme, this woman had an unusual difference:
she expressed multiple copies of the gene. As a result, the
additional CYP2D6 enzymes in her body churned through the
codeine so efficiently that her morphine levels spiked and
stayed high. Unbeknownst to her doctors, high doses of the drug
had passed from her milk into her infant. When the baby was
brought back to the hospital with gray skin 12 days after his
birth, doctors were puzzled. He was an otherwise healthy baby
boy. The next day he died—the concentration of morphine in his
blood 30 times higher than expected. The case highlights not
only how variable adults can be in how they metabolize drugs,
but also how challenging it is to correctly diagnose and treat
infants, whose symptoms and basic physiolo