Even for the severe pain of kidney stones, minorities were prescribed narcotics such as oxycodone and morphine less frequently than whites.

The analysis of more than 150,000 emergency room visits over 13 years found differences in prescribing by race and ethnicity in both urban and rural hospitals, in all US regions and for every type of pain.

"The gaps between whites and nonwhites have not appeared to close at all,'' said study co-author Mark Pletcher of the University of California, San Francisco.

The study appears on Wednesday’s Journal of the American Medical Association. Prescribing narcotics for pain in emergency rooms rose during the study, from 23 per cent of those complaining of pain in 1993 to 37 per cent in 2005.

The increase coincided with changing attitudes among doctors who now regard pain management as a key to healing.

Doctors in accredited hospitals must ask patients about pain, just as they monitor vital signs such as temperature and pulse.