People taking opioid painkillers face higher risks of car accidents even at low doses, say Ontario researchers who want patients to be warned that the drugs can decrease alertness.

Knowing that use of opioids like oxycodone, codeine and morphine has increased in North America and that driver simulation studies suggest that the drugs hinder alertness and act as a sedative, researchers at Toronto's Institute for Clinical Evaluative Sciences looked at emergency department visits among adults treated with opoids.

They defined road trauma as motor vehicle crashes that required a visit to emergency.

The increased risk for drivers taking opioids started with the lowest doses equivalent to 20 milligrams of morphine. (iStock)

Compared with very low doses of opioids, drivers prescribed low doses such as 20 milligrams of morphine showed 21 per cent increased odds of car accidents which rose to 42 per cent for those prescribed high doses, Tara Gomes and her co-authors reported in Monday's issue of the JAMA Internal Medicine, formerly Archives of Internal Medicine.

"What was surprising to us was this increased risk started even at what many people consider to be fairly low doses of opioids," Gomes said in an interview.

Gomes suggested that when people are prescribed an opioid, particularly as the dose increases, they should consider avoiding driving if possible and consider asking a friend or relative to drive them while completing the drug treatment. Doctors may want to warn patients about the potential for reduced driving ability, she added.

In public health terms, there could be hundreds of thousands of people taking these medications and driving while their ability to operate the vehicle safely is affected, she said.

The researchers initially looked at prescription databases and emergency department records for more than 500,000 patients aged 18 to 64. From that group, they compared 5,300 cases of traffic collisions with an equal number of controls the same sex and in the same age group who took opioids for the similar time periods but never went to an emergency trauma for crashes.

Of the cases, nearly 46 per cent were drivers. They did not find an association between drug dose and risk of collisions for passengers, pedestrians or others injured in traffic collisions.

"This study is the first, to our knowledge, to demonstrate the relationship between opioid dose and this risk among drivers in a population-based setting," the study's authors concluded.

"Policy makers could improve public education surrounding the potential risks of opioid medications and could consider restricted drivers' licences for patients treated with high-dose opioids."

The study included only those eligible for public drug coverage, such as those receiving social assistance. Gomes said it's unlikely that their driving would be affected differently than the general population.

When opioids are prescribed, doctors should weigh the risk of road accidents into their decisions about the benefits and harms of the drugs, Dr. Mitchell Katz of Los Angeles Country's department of health services said in a journal editorial accompanying the study.

Pain relief is a core part of a physician's mandate, and it's tempting to prescribe higher doses to patients with chronic pain, but doctors should resist doing so, Katz said.

"In the case of opioids for chronic pain, for the sake of our patients, less is often more," Katz concluded.

In the study, the risk of road accidents among those prescribed high doses of opioids was 42 per cent higher than those prescribed very low doses. But in the highest dose category, the risk was 23 per cent higher.

The researchers don't know why that is. They speculated that those in the very high dose category could be selling the drugs, which Gomes said are highly sought for their addictive properties, or that these patients developed tolerance which reduced the impact on driving.

The study was funded by the Ontario Ministry of Health and Long-term Care and ICES.

With files from CBC's Amina Zafar