Dr. David Juurlink is shown in a 2011 photo. Juurlink took part in a study looking at the use of opioids to control pain and the risk of motor vehicle accidents. THE CANADIAN PRESS/HO, Doug Nicholson

TORONTO - Taking even low doses of opioids like codeine or morphine increases the risk of being involved in a traumatic motor vehicle accident — and that risk goes up as people take higher and higher amounts of the powerful pain killers, research suggests.

Opioids, the widely prescribed class of drugs that also includes oxycodone, hydromorphone and the fentanyl patch, are prescribed for both acute and chronic pain. The medications, especially immediately after they are taken, can play havoc with a person's alertness and reaction time.

The study, published Monday in JAMA Internal Medicine, found that among adult drivers prescribed opioids, a daily dose of just 20 milligrams of morphine (or the equivalent of another opioid) was linked to a 21 per cent increased risk of road trauma.

The chance of having an accident rose to 42 per cent with high doses of an opioid, described as 100 to 199 mg of morphine or its equivalent, compared to low doses of the drug.

"What we found was that the risk of a motor vehicle collision was dependent on the dose of opioid the individual was receiving, this was for people who were driving the car," said Dr. David Juurlink, one of the co-authors of the study by the Institute for Clinical Evaluative Sciences in Toronto.

"I think the most important observation was that even low doses of opioids — these are doses in the 20 to about 50 milligrams or so per day — were associated with an increased risk of accidents compared to very low doses," he said.

Meanwhile, those prescribed very high doses of an opioid — more than 200 milligrams per day — also had an increased risk of road trauma while driving. But at 23 per cent, it was lower than might be expected.

Despite the high-end dose, Juurlink said the risk may be lower because patients taking that level of drug have likely been on the medication for a while and their brains have adjusted to its sedating side-effects.

Another possibility is that people on very high doses drive less because they're in severe pain, so it only appears that the risk for this group is lower, he said. "It's also conceivable that some people are not taking but actually selling (the drugs), so that also might attenuate the risk to a certain extent."

Using Ontario prescription data and hospital records, the study looked at almost 550,000 patients aged 19 to 64 who received at least one publicly funded prescription for an opioid from April 2003 to March 2011. The study examined 5,300 of these patients who were treated in a hospital emergency department after being involved in a traffic accident.

Being in pain can in itself hurt a person's ability to drive, depending on what part of the body is affected and how severe it is, said Juurlink.

"But these drugs impair alertness. They can make you sleepy. Anyone who's taken them knows that can happen," he said. "They make you less attentive and they can interfere with your motor responses or the quickness with which you respond to a threat.

"So there are many ways in which opioids and other drugs for that matter can impair one's ability to drive."

While risk levels of 21 to 42 per cent may seem modest at first glance, widespread prescribing of opioid drugs means the issue of road safety is an important one, the study authors stress.

"When you think about the hundreds of thousands, if not millions, of Canadians who take these drugs, I think it's fair to say that the use of opioids while driving may constitute a significant and previously under appreciated public health problem," Juurlink said.

Patients taking the pain killers should consider hanging up their car keys or at least minimizing the amount they get behind the wheel, he said, and physicians can also play a role in keeping these drivers safer, as well as their passengers, pedestrians and other motorists.

"I think it's incumbent upon docs to warn patients at the time a prescription is written that they should think twice about driving."

News from © The Canadian Press, 2013