Whites more likey to receive narcotics?

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A study has claimed that emergency room doctors are prescribing strong narcotics more often to patients who complain of pain and whites. The study incorporates information from over 150,000 emergency room visits over a 13 year time period. It goes on to say that this trend is the same in all kinds of hospital - urban, rural, all regions, and for every kind of pain. One of the authors pointed out that the gap between whites and minorities is large. I bring this up not because I'm racist or biased in favor or against another, but this is interesting. What do you think is the cause of this?