We already know that opioids are not only dangerously addictive but often don’t provide long-lasting relief for painful chronic conditions.

Now we have an insight into how these painkillers backfire—and how quickly that can happen.

In a new study, animals in pain who received morphine, an opioid, became more sensitive to pain.

And that’s not even the worst part.


In a study of four groups of rats, two groups were experimentally injured while the others were given a painless procedure. One set of injured rats and one set of unharmed rats were then given morphine, while the other animals were injected with saltwater. Each treatment lasted five days, and then the animals were exposed to two other painful stimuli—an electrical shock and mechanical pressure.

Whenever nerve cells are damaged, they send distress signals via glial cells in the brain, and you perceive pain. But after morphine, the researchers found, those pain-activated glial cells became more sensitive to the next pain stimuli. As the researchers put it, “Opioids exaggerate pain.”

The worst part: It took months for the effect to wear off.


The medical community is finally realizing what a big mistake it was to turn so readily to opioids for management of chronic pain—for conditions such as fibromyalgia, for example. Although opioids are still widely overprescribed for chronic pain, the latest CDC guidelines state that they should be avoided if possible, with the exception of cancer pain and end-of-life palliative care. Emerging research is also finding new harm from long-term opioid use, including an increased risk for heart disease.

The trickier question is how opioid use for acute pain can turn into long-term dependency. While animal studies such as this one don’t tell us exactly how these drugs work in humans, the new findings shed some light on how treating acute pain with opioids might lead to the development of chronic pain—by prolonging pain sensitivity.

It’s common for doctors to prescribe opioids after an operation, injury or dental procedure. Sometimes they are needed. But if you can use non-opioid medications instead…along with physical therapy, exercise, ice and other approaches—you can avoid the risk for this rebound effect.

For nondrug approaches to chronic pain, see Bottom Line’s article, “Live Pain-Free Without Drugs.” To learn more about dependency, including how to get off opioid medications if you think you have a problem, see “Flirting with Painkillers: Could You Get Hooked?