In many countries you don't need a scrip to buy low grade opioids like codeine. That will inflate somewhat the US number.
> These data represent the amounts of opioids distributed legally in a country for medical and scientific purposes to those healthcare institutions and programs that are licensed to dispense to patients, such as hospitals, nursing homes, pharmacies, hospices and palliative care programs. Consumption does not refer to the amounts dispensed to, or used by, patients, but rather to amounts distributed to the retail level.