The surprising findings not only point to novel future treatments for pain but reveal an unexpected association between chronic pain and lung cancer.

"Chronic pain is currently subjected to often ineffective palliative treatments," said co-corresponding author Clifford Woolf, explaining the origins of the research. "Furthermore, effective painkillers such as opioids can lead, if used inappropriately, to severe addiction. It is therefore critical to find and develop new and repurposed drugs to treat chronic pain."

Prior investigations from the researchers led to the discovery of a particular signaling pathway unique to chronic pain. They found a metabolite called BH4 is produced by sensory neurons in cases of neuropathic and inflammatory pain.

High levels of BH4 had been linked to severe pain intensity in chronic pain cases, so the next step was to look for drugs that could block BH4 signaling in pain neurons.

To do this, the researchers performed a phenotypic screen on around 1,000 known bioactive molecules. They were looking for drugs that could regulate the expression of the gene linked to higher levels of BH4.

Several currently used medicines popped up in the screening study, including drugs like clonadine that are known to generate analgesic effects. But most interesting was the discovery that an old anti-psychotic drug called fluphenazine seemed to work on this particular pain-signaling pathway.