TORONTO -- Medically sanctioned opioid use has dropped by almost 14 per cent since national guidelines for prescribing the drugs were introduced in 2010, yet the rate of overdose-related hospital visits continued to rise, an Ontario study has found.

The study found opioid-related hospitalizations in the province rose 13 per cent between May 2010 and the end of 2013, possibly because people who continue to take these addictive narcotics for chronic non-cancer pain were often prescribed them in high doses, placing them at greater risk for overdose.

To conduct the study, researchers examined health data for people who were eligible for the Ontario Drug Benefit program between 2003 and 2014, identifying almost 770,000 who were dispensed at least one opioid prescription during that period.

"We looked specifically at hospital visits for opioid overdoses and found that the rates of those visits had continued to climb over that same period and don't appear to have been impacted by the guidelines," said Tara Gomes, a scientist at St. Michael's Hospital in Toronto, who led the research.

Those guidelines suggest clinicians exercise extreme caution when prescribing an opioid at a dose that exceeds 200 milligrams per day of morphine or an equivalent, such as oxycodone or fentanyl.

Yet the study, published Wednesday in the journal PLOS ONE, found a large number of patients were being prescribed at that 200-milligram level and some were taking double that potency.

Despite falling rates of opioid prescribing, the proportion of high-dose use among the remaining users more than doubled over the study period -- from 4.2 per cent in 2003 to 8.7 per cent in 2014.

Researchers found 41 per cent of recipients of long-acting opioids exceeded daily doses of 200 mg of morphine, or an equivalent, and almost 19 per cent were treated with very high doses exceeding 400 mg.

"What's incredible to me is we're seeing about 75 per cent of fentanyl users getting over 200 milligrams of morphine equivalent," said Gomes, referring to the prescribed fentanyl patch, not the pills sold illicitly on the street that have caused so many overdose deaths in Canada this year.

Because opioids affect the central nervous system, taking too high a dose can severely depress a person's respiration, leading to unconsciousness and death.

"So what you often hear when people die of an opioid overdose is that they fall asleep because they've been sedated by these drugs, and then their breathing becomes very shallow and they just stop breathing and die," said Gomes.

So why do doctors keep prescribing these drugs? And why in such high doses?

Gomes believes that once a patient becomes reliant one of the narcotics and needs increasingly higher doses to maintain its effect, it's difficult for a physician to taper down their dose or have them stop taking it altogether.

"What we've seen over the last decade or so is more and more people getting access to these drugs, escalating doses, and now in the last couple of years we've really started to get the message out that there are serious risks around high-dose opioid use," she said.

"But it's clearly not having an impact on the dose prescribed, which might just be because prescribers are having a very hard time getting their patients off these drugs."

Gomes said policy makers and health practitioners need to do more to turn the tide of opioid overprescribing and the epidemic of overdoses, including making sure patients have access to addiction services.