Morphine can more than double the duration of pain and increase its severity, says a new animal study.
"Our results add weight to the growing body of science suggesting that treatment with opioids such as morphine may in fact be a contributor to people's chronic pain," says study author Dr Peter Grace from the University of Colorado Boulder and the University of Adelaide.
But the researchers also said they discovered how to switch off the pain-amplifying mechanism which would enable morphine's painkilling benefits to be delivered without the drawbacks.
The study, published in the journal Proceedings of the National Academy of Sciences, found that rats with chronic nerve pain treated with morphine for five days experienced prolonged pain sensitivity.
This lasted for up to two-to-three months, more than double that experienced in the control group.
"What we found is that the opioid painkiller activates spinal immune cells, causing a further inflammatory response," said Dr Grace.
"The pain is effectively transitioned to a chronic state, making the pain itself both more severe and longer lasting."
Dr Grace says the results are of concern because of the huge number of opioid prescriptions being written for patients each year.
Recent research by The George Institute found that opioid painkillers, even at high doses, provide minimal benefit for people with lower back pain.
And global public health authorities have expressed concern about the increased use of the powerful drugs leading to addiction and abuse, as well as overdose deaths.
