Earlier this year the FDA issued a warning and changed the label on pain medications that contain codeine because of unpredictable serious adverse effects including death in children.
Codeine is metabolized into morphine and some children are genetically predisposed to be "ultra-rapid metabolizers". Some of these children have had fatal or life-threatening morphine overdoses after taking typically safe codeine doses for post-op pain management after tonsillectomy.
The most common codeine-containing medication is acetaminophen with codeine (aka Tylenol #3).
The FDA now "strongly recommends" against codeine use in post-op tonsillectomy pain management.