You’d be forgiven for expecting, post-surgery, to be given a nice big hit of painkillers. Well, new research is suggesting that prescribing someone with opioids might not be such a good idea, and could instead be prolonging chronic back pain, not making it better.

“There is surely a dark side in terms of addiction when it comes to opioids, but this is a very different idea – that we think we are treating the pain with these drugs and we may actually be prolonging it,” said Linda Watkins, who is the senior author of the paper exploring this effect, published in the journal Anesthesia & Analgesia.

The team delved into how these drugs might be impacting chronic pain by looking at the effect it had on rats. First of all, they unfortunately had to perform exploratory abdominal surgery on the rodents as a proxy for a similar procedure that is performed on tens of thousands of Americans every single year, which is then more likely than not followed up by a prescription of opioids.

The rats were then divided into three different groups. In the first, half the rodents were given a “moderate” dose of morphine for a period of seven days, and the other half a simple saline solution. In the second cohort, half the rats were given the opioids for eight days, and then had it tapered off by the 10th day, and in a final experiment, the rats were given full doses of the drug until day 10, at which point it was abruptly cut off.

The rats were tested as to how they responded to touch, and had their gene activity monitored to see if there were any changes in the expression of inflammatory proteins in the spinal cord.

Those rats given the drugs were found to experience postoperative pain for up to three weeks longer. Not only that but it turned out that the longer the rats were on morphine, they longer they expressed signs of chronic back pain, and that gradually cutting the amount of opioids over a period of time seemed to make no difference to this.

The researchers suspect this might have something to do with the way the body responds to pain, and that the drugs are effectively priming the immune system to respond to pain. They think that in response to the surgery, the immune system is basically put on guard and a cascade of inflammatory proteins is released, in case there is a second hit. The morphine administered directly following the surgery is thought to be this second hit.

“With that second hit, the primed glial cells respond faster, stronger and longer than before, creating a much more enduring state of inflammation and sometimes local tissue damage,” explained Watkins.

Obviously, these experiments were only conducted on rats so the conclusions that can be drawn are limited, but the researchers now hope to explore whether or not the effect can be seen in humans, too, and whether this means that people are more likely to stay on the drugs.