TORONTO -- A study has found the use of prescription opioids varies dramatically across Ontario, but overall the potent and addictive drugs are responsible for hundreds of overdose deaths in the province.

The study by researchers 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.

Lead researcher Tara Gomes says coroners' reports show almost 13 per cent of those overdose deaths were suicides.

Gomes says the Thunder Bay District and Timiskaming District had the highest rates of opioid-related deaths in Ontario, about four times the provincial average.

The researchers found there were 3,200 opioid-related emergency department visits in Ontario in 2014, which resulted in about half of those patients being admitted to hospital.

"This report illustrates the considerable variation in both prescribing and adverse events across the province," said Gomes. "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 overdose from such opioids as oxycodone, hydromorphone and fentanyl.

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

"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."