The study finds that codeine is prescribed for children in at least half a million emergency room visits each year.
It's not rampant. The 10-year study finds just 3 percent of kids' ER visits resulted in a codeine prescription in 2010. But with more than 25 million ER visits by children each year, the authors say that's still too much.
Codeine is an opiate and a genetic variation makes some people metabolize it too quickly. The Food and Drug Administration issued its strictest warning last year about a rare risk for life-threatening complications or death in children.
Another genetic variation makes the drug ineffective for pain relief in as many as a third of patients.