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New Study Suggests Opioids Could Make Chronic Pain Worse

A new animal trial suggests that opioid medications, such as morphine, could be causing an increase in chronic pain. The findings indicate that opioids, which are heavily prescribed in the U.S. and already under scrutiny because of their abuse and overdose risk, may be elongating chronic pain in humans.

Although commonly prescribed for the treatment of chronic pain, opioid medications may actually be making chronic pain worse, findings in a new study suggest. In the study, published in the June issue of Proceedings of the National Academy of Sciences, researchers at University of Colorado Boulder discovered that opioid treatments intensified chronic pain in lab rats.

After just five days of morphine treatment, the release of pain signals from specific immune cells in the spinal cord were found to be exacerbated, causing chronic pain in the rats to go on for several months. The results suggest that opioid painkillers, which are the regular pain-relieving go-to for physicians, may actually be causing chronic pain rather than treating it.

Researchers mimicked chronic nerve pain in a group of rats by opening the animals’ thighs and tying a thread around a nerve. Ten days later, half of the injured rats were treated with morphine for five days. All rats were tested for pain sensitivity over the next three months. After just six weeks, the rats not treated with morphine were found to have the same pain sensitivity as rats that were not injured at all. The rats treated with morphine, however, needed 12 weeks to see their pain sensitivity return to the level of an uninjured rat.

“Our key finding is that we were able to demonstrate that a brief treatment with a pain killer, like morphine, doubled the duration of chronic pain,” said Peter Grace, a CU assistant research professor and one of the leaders of the study.

Grace and his co-researchers found that following a nerve injury, a message is sent from the damaged cells to the spinal cord immune cells. These cells, known as glial cells, are responsible for clearing out unwanted debris and microorganisms. Pain signals put the glial cells on alert and prime them for action. The morphine was found to intensify the pain signals from the glial cells, leading to spinal cord inflammation and prolonged pain.

“Initially the morphine does work and it produces great pain relief and we know that over time the ability for morphine to produce analgesia or pain relief diminishes over time,” added Grace. “And what we’re showing is rather than alleviating pain it’s actually inducing it.”

“The implications for people taking opioids like morphine, oxycodone and methadone are great, since we show the short-term decision to take such opioids can have devastating consequences of making pain worse and longer lasting,” said Professor Linda Watkins, co-author of the study. “This is a very ugly side to opioids that had not been recognized before.”

The study’s findings identify a possible new peril associated with opioids, which have already been attributed to abuse, addiction, and overdose problems. The United States is in the midst of an opioid abuse epidemic, as over 1.9 million Americans battled a substance abuse disorder involving painkillers in 2014, and nearly 19,000 of those ended in death.

However, an April survey found that medical marijuana helps patients significantly reduce their intake of opioid pain medications, thereby decreasing their risk of abuse and overdose. Last fall, a team of researchers published an editorial urging physicians to treat their chronic pain patients with cannabis rather than opioids, arguing that cannabis is safer and more effective for pain relief.