We know that pain thresholds vary from person to person – one person’s nudging inconvenience is another’s unbearable distraction – but the colour of your skin, in the US at least, can be a factor in deciding whether or not you receive pain medication.

New research from the US shows that black patients who arrive at emergency rooms complaining of back or abdominal pain are significantly less likely to be given opioid painkillers, such as codeine, than their white counterparts, even when pain levels and insurance coverage are the same. Pain is the most common reason Americans visit the ER, and the researchers in Boston looked at five years’ worth of records across the country for patients who had complained of general pain with an unclear cause.

This effect has been found across healthcare settings – black children with appendicitis are significantly less likely get any painkillers for moderate pain or opioid-type drugs for severe pain. Doctors from the University of Pennsylvania reviewed 20 years’ worth of studies in 2012 and found the same, calling this “the treatment gap”. The review’s lead author, Dr Salimah Meghani, says that to stop the disparity, doctors need to be educated about its cause, and this is most likely unconscious bias. The problem, she says, “may be addressed by helping health providers identify their blind spots”.

In April, another study found that half the white medical students surveyed believed at least one false statement about black people having “thicker skin”, “faster-coagulating blood” or “less sensitive nerve endings”. Medics who believed one of these facts also rated black pain lower when viewing case studies. Authors of this research believed it was down to these entrenched ideas about “biological differences” between races that may be informing doctors’ decisions on when to prescribe.

Dr Austin Leach, a pain medicine consultant who serves on the council of the British Pain Society, says there is no biological reason for different prescriptions. “Physiological studies suggest that pain perception is the same across ethnicities,” he says, adding that there is no evidence that this happens in the UK. Some races are predisposed to certain illnesses – the rate of strokes for African Americans is double that of the country’s white population – but the treatment of pain should be the same.

In this latest study, researchers claim that a particular type of unconscious bias – assuming that black patients are abusing meds – is to blame, despite the fact that opioid abuse is more prevalent in white communities.

The study’s lead author, Dr Astha Singhal, a professor at Boston University, says the key point is that they found a discrepancy only in vague pain-related conditions, commonly associated with drug-seeking behaviour, rather than conditions with obvious causes, such as bone fractures. “While the disparity we found is surely not a good thing, it might have indirect advantage to the black patients of minimising their exposure to opioids,” she says.

The theory that black communities are being “shielded” from opioid addiction by not being prescribed the drugs in the first place is a controversial one. Other studies have shown that diagnosis of issues from back pain to cancer is slower for black patients. The line is unclear to many doctors, including Singhal. “It raises the question – is it still an advantage if the cost is bearing untreated or undertreated pain?”