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Prescription Painkillers Could Make Pain Last Longer

A study in rats hints that morphine and other opioids trigger inflammation and, as a result, more pain in the long term.

By Nathan Collins


(Photo: tippa/Flickr)

Prescription painkillers can cause a lot of harm. Their abuse led to a strange outbreak of HIV in rural Indiana last year, and they probably played a role in Prince’s death. On a larger scale, the rise in painkiller abuse and deaths led the Centers for Disease Control and Prevention to issue recommendations against prescribing the drugs for chronic pain. If that isn’t enough to make you think twice about treating chronic pain with prescription opioids, there’s also this: doing so may actually make your pain last longer, according tonew research.

Researchers still don’t know too much about the effects of opioids like oxycodone, one of two main ingredients in Percocet, on people in the long term. “Recent reports are critical of the lack of controlled, long-term studies to support the dramatic escalation of opioid treatment for chronic pain over the past decade,” University of Colorado-Boulder research assistant professor Peter Grace and his colleagues write in Proceedings of the National Academy of Sciences. “Although one long-term concern is that there may be no benefit, another is that opioid treatment could have negative consequences for pain.”

Morphine triggers a particular immune inflammatory response, which inadvertently helps turn a painful injury into chronic pain.

To start to get a handle on the question, Grace and his colleagues first tied little bits of thread around rats’ sciatic nerves, inducing, as one might expect, some chronic pain. Starting 10 days later, half the rats received a five-day course of morphine for the pain, while the others got salt water. The researchers then performed a variety of tests, including one in which they tickled the rats’ hind paws and looked at how hard they pulled away.

Judging by the paw test, rats who received morphine experienced nerve pain about twice as long as rats who got no opioids—they started to recover at about eight weeks, as opposed to four. Rats who had been treated with morphine also recoiled much harder than those who hadn’t, suggesting their pain was greater after they went off morphine, compared with rats who’d never taken the drug.

Additional tests pointed to a surprising culprit—morphine triggers a particular immune inflammatory response, which inadvertently helps turn a painful injury into chronic pain. That observation could help researchers identify drugs to treat acute pain without the long-term consequences, Grace and his colleagues write.