Disability claimants who are off work because of low back pain stay off longer if they're being treated with opioids, according to a new study out of McMaster University.

The study, which was published Thursday on BMJ Open, highlights the dire need for controlled trials on the use of opioids to better understand the powerful prescription painkillers that are gaining notoriety for their addictive effects, says lead author Jason Busse.

"We have no gold standard evidence to guide treatment in this area," said Busse, an assistant professor of anesthesia and researcher with the Michael G. DeGroote Institute for Pain Research and Care.

But that hasn't stopped doctors from prescribing opioids like Oxycodone and Fentanyl. A recent study published in Canadian Family Physician says Canada and the U.S. now have the highest levels of prescription opioid consumption in the world – and that number continues to rise dramatically.

"Canada is now one of the leading per capita prescribers of opioids in the world," Busse said. "We're very generous with our prescribing in Canada."

Multiple studies, same conclusion

For the study, Busse and his team analyzed 1,442 claims for acute, uncomplicated lower back pain that were approved by the Ontario Workplace Safety and Insurance Board (WSIB) in 2005 that remained on full benefits at four weeks after claim approval.

They found that people who were prescribed opioids stayed off work longer. Those findings come at a "time of crisis" for the WSIB, Busse said. In order to manage a "growing unfunded liability" that topped $13 billion in 2013, the Ontario WSIB has increased their claim denial rate, decreased benefits to injured workers, reduced WSIB staff, and raised employer premiums, he said.

To bolster their findings, the McMaster researchers also reviewed three other studies, which all found similar findings. However, Busse is careful to caution that their evidence doesn't equal causation.

Workers who were prescribed opioids might have been legitimately worse off, and that's why they were off longer, and were prescribed the painkillers. "On the other hand, adverse effects of opioids on mental and physical function could be the reason," Busse said.

There's only one way to know for sure, and that's with definitive research studies – ideally randomized trials.  But near as McMaster's researchers can tell, those don't exist. "We were very surprised to find that gap," Busse said.

Researchers did find a number of clinical trials on opioids, but they tended to exclude patients who are receiving disability benefits. That's because they would likely produce negative results, Busse said.

"And certainly, a lot of these trials are industry funded," Busse said. "It is very tempting to exclude patients from studies who wouldn't do very well," he said, adding that someone like Health Canada needs to step up organize that sort of trial.

Painkillers linked to hundreds of deaths

Dr. David Juurlink — a Sunnybrook Health Sciences Centre staff physician who has extensively studied painkiller prescribing practices — agreed with Busse's calls for randomized trials. "But randomized trials are no small business," he said.

It can take about $2 million to have someone "run with that football," and drug companies have no incentive to do that because they are only concerned with getting their product into the marketplace, he said. "It's a herculean task," he said. 

And while studies on the subject are lacking, people are dying. According to a recent study from The Canadian Community Epidemiology Network on Drug Use, the powerful prescription-only painkiller fentanyl was involved in 655 deaths between 2009 and 2014. A study by Juurlink found that almost four per cent of men who were taking high dose opioids died of opioid related causes.

"That's just a staggering number," he said in a previous interview with CBC News. "What that means is, if you're a man taking opioids at high doses, you're more likely to die from your medications than almost anything else."

"I think we need to think long and hard before we start a patient on opioids."

adam.carter@cbc.ca