The percentage of postpartum women who filled codeine prescriptions significantly declined after regulatory bodies announced that breastfeeding by mothers taking the opioids may cause infant death, according to a study.

Infants who are breastfed are at elevated risk of death, if their mothers carry polymorphisms for increased activity of an enzyme that metabolizes codeine to morphine, Kate Smolina, Ph.D., and her colleagues reported in a research letter in JAMA.

The study included 320,351 live births to 224,532 women in British Columbia, Canada, between January 1, 2002 and December 31, 2011. The researchers studied dispensation rates of codeine and other opioid drugs to women during the first 6 months after giving birth. New mothers’ use of cephalexin and hydrocortisone cream, which are commonly prescribed to postpartum women, served as the control measure. The announcements on the risks of breastfeeding while taking codeine were made by the U.S. Food and Drug Administration in August 2007 and Health Canada in October 2008. Prior to the FDA announcement, on average, nearly 17% of the postpartum mothers filled at least one codeine prescription. Between September and December 2011, the monthly average was down to 9% – representing a relative reduction of 45% over 4 years. Postpartum use of the opioids tramadol, hydromorphone, oxycodone, and morphine increased, but not at a large enough rate “to offset the reduction in codeine use,” according to the researchers.

The rates of prescription filling did not significantly change for cephalexin and hydrocortisone cream.

Read the study in JAMA 2015 May 12 ( doi:10.1001/jama.2015.3642 ).