The use of opioids varies dramatically across Ontario, but overall the potent and addictive medications are responsible for hundreds of overdose deaths each year, researchers say.

A study by scientists at the Ontario Drug Policy Research Network found 638 people died in 2013 from opioid overdoses -- a rate of about one death for every 20,000 residents in the province. The report, released Thursday, found 13 per cent of those fatal overdoses were suicides.

In the five years ending in 2013, the most recent year for which statistics are available in Ontario, there were 2,879 deaths from opioid overdoses, the study found.

"I think it highlights that this issue is not going away," said lead author Tara Gomes, a researcher at St. Michael's Hospital in Toronto. "We continue to see high rates of fatal and non-fatal overdoses and a large degree of opioid prescribing across the province."

The report shows that the Thunder Bay and Timiskaming districts had the highest incidence of opioid-related deaths, about four times the provincial average, though it's not clear why fatality rates in some areas of the province differ from others.

"It becomes really difficult to understand what is driving those patterns," admitted Gomes. "Is it differences in certain prescribers that are perhaps more broadly prescribing these drugs ... (or) is it perhaps that there's more illicit opioid use in some of these areas ... which would certainly contribute to the overdoses and deaths that we are seeing."

Not all opioid overdoses end up being fatal, but they have a significant impact on those taking the drugs and on the health-care system, the report found.

Tapping into 2014 hospital records, researchers identified about 3,200 opioid-related emergency department visits in Ontario, which resulted in 1,620 patients being admitted to hospital.

"It starts to show that the deaths are really the tip of the iceberg when we get down to these overdoses that are ending up in hospital," said Gomes.

The highest rates of opioid-related emergency department visits and hospitalizations occurred in health units in northern Ontario as well as in parts of southwestern Ontario, specifically the Erie St. Clair and Hamilton-Niagara-Haldimand-Brant regions.

While some overdoses may be linked to the use of illicit opioids such as heroin, experts say overprescribing by doctors is the main driver of people using and becoming addicted to such drugs as oxycodone, hydromorphone and fentanyl patches -- and accidentally or deliberately taking a deadly dose.

"These findings highlight the massive societal toll of opioid-related morbidity and mortality," co-author Dr. David Juurlink, head of pharmacology and toxicology at Sunnybrook Health Sciences Centre, said in a statement.

"Prescribed with care, opioids can help some people, but it's essential that patients and doctors recognize the potential harms associated with use of these drugs."

However, the researchers found prescribing and use of the potent narcotics was widespread.

Using 2015 data from the Ontario Drug Benefits Program (ODBP) -- which covers the cost of drugs for low-income residents and seniors, for example -- the study shows almost 670,000 people had filled prescriptions for an opioid for pain, while almost 34,000 were dispensed a drug such as suboxone for opioid maintenance treatment as part of addiction therapy.

"We know these are huge underestimates of the degree of opioid prescribing in the province," said Gomes, noting that the ODBP database doesn't capture those who pay for the medications from their own pockets or through private insurance plans.

However, dispensing rates under the publicly funded drug plan tend to mirror overall prescribing patterns in Ontario, she said.

Still, Gomes stressed the report turned up "considerable variation in both prescribing and adverse events across the province," which should help individual health units better target interventions.

"This information can be used to identify regions with the highest need for programs and services to address opioid addiction and overdose, such as addiction services, safe-injection sites and access to naloxone."

Naloxone is a rescue medication that can reverse the effects of an opioid overdose.