Morphine, far from being a panacea, might actually prolong patients' pain and make it worse, new research suggests.
According to a study published today in the journal Proceedings of the National Academy of Sciences, rats suffering nerve pain given morphine for five days ended up taking twice as long to recover -- two to three months -- than others.
"The opioid painkiller activates spinal immune cells, causing a further inflammatory response," says study author Dr Peter Grace of the University of Colorado.
"The pain is effectively transitioned to a chronic state, making the pain itself both more severe and longer lasting."
Dr Grace says the findings add to growing evidence that opioid painkillers -- which include methadone, codeine, heroin and oxycodone -- are only a temporary fix for chronic pain.
It follows a 2013 study looking at the effects of codeine, one of the most popular painkillers in the world, found it provides much less pain relief than morphine but resulted in a similar increased sensitivity to pain in the long-run.
The new research, however, has found a potential way to turn off the spinal cells reacting badly to opioids.
"We've been able to block the two main receptors involved in this immune response...which have both been separately implicated in chronic pain before," says Dr Grace.
"By blocking these receptors, we're preventing the immune response from kicking in, enabling the painkilling benefits of morphine to be delivered without resulting in further chronic pain."
Because it's only been tested on rats so far, it's likely to be at least five years before patients stand to benefit, if the treatment's deemed safe in human trials.