African-Americans who seek care for back or abdominal pain in the nation's emergency rooms are less likely to receive opioid pain relievers than white patients who presented with the same symptoms.

That's the finding of a new study conducted by researchers at the University of California at San Francisco. Published Aug. 8 in PLOS ONE, the study looked at five years' worth of data from the National Hospital Ambulatory Medical Care Survey.

Researchers reviewed 36.5 million emergency department visits from 2007-2011. They looked for visits related to specific (long-bone fractures and kidney stones) as well as nondefinitive symptoms (toothache, abdominal pain and back pain).

The majority of patients who presented with any of the symptoms received opioids, either in the ED or as a prescription at discharge. More patients with a definitive condition than a nonspecific complaint received opioids (80-85 percent compared to 52-65 percent, respectively).

However, a significant difference was made clear when the research team adjusted the numbers to account for race. Blacks with nondefinitive pain were half as likely as whites to receive opioids either in the ED or upon discharge. Other minorities were also less likely to receive opioid pain medications for abdominal pain.

There were no statistical differences when people of any race presented with definitive symptoms like broken bones or kidney stones.

However, as the study authors point out in the discussion, whites are more likely to be affected by prescription opioid abuse.

"Recent evidence shows that non-Hispanic whites are disproportionately affected by the prescription opioid epidemic," the authors wrote. "While non-Hispanic blacks do not benefit from bias, they might be inadvertently benefiting by receiving fewer opioid medications and prescriptions."

In fact, statistics indicate 38,000 white Americans and 4,000 black Americans died of an opioid overdose in 2014.

Though some may call it a "silver lining," the study presents clear evidence that whites and blacks are treated differently.

"We may agree that opioids can be harmful and that fewer of them may be a good thing," Adam Hirsh, a pain researcher at Indiana University, told The New York Times. "But we should not ignore that black and white patients are getting treated differently."

The study's authors agree. The statistics show the need for sensitivity training among emergency physicians, authors say.

"Whether or not someone 'needs' opioids is a very subjective decision  —  there isn't an objective test," said Dr. Renee Y. Hsia, a professor of emergency medicine at UCSF and the senior author of the study. "Even when we are trying our best to care for our patients, bias can creep in. When we actually look at the data on how medical professionals prescribe opioids, we see differences."

Multiple studies in recent years have looked at the differences between blacks and whites when it comes to prescription pain medication.

An April 2016 study by the University of Virginia revealed bias among doctors who believed blacks had higher tolerances for pain. A 2015 study showed that white children with appendicitis received more potent pain medication than black children. And a 2013 study revealed that not just blacks, but poor people were less likely to be prescribed opioids in the ED setting.