The drugs, known as opioids, include common painkillers like codeine and oxycodone.

High rates of opioid prescribing have become more of a concern in recent years, as deaths from overdoses rise and more drugs end up in the hands of people taking them for non-medical purposes.

Their effect on road safety may be one more reason to think carefully about high-dose opioids, researchers said.

"Even what people may consider to be low or moderate doses of opioids can have substantial impact on your risk of being involved or injured in a motor vehicle accident, and I think patients should be informed about that," said Tara Gomes, who led the study at the Institute for Clinical Evaluative Sciences in Toronto.

Gomes said previous small, controlled studies have suggested people taking opioids may have trouble with alertness and sedation while driving. But she and her team wanted to take a wider look at the public health implications of driving while on painkillers.

They compared 5,300 Canadians who came to the emergency room after a car accident and were taking at least one opioid with a similar number of people who were also on painkillers, but hadn't been in an accident.

The researchers determined how much of an opioid drug someone was taking using a prescription drug database for Ontario residents who got help paying for their medicines.

The team found that overall, there was no difference in the number of traffic injuries among people taking low- or high-dose opioids, according to findings published this week in JAMA Internal Medicine.

However, when the researchers focused just on drivers, they found that people taking moderate-dose opioids were 29 percent more likely to be injured in a crash while behind the wheel and those on high-dose opioids were 42 percent more likely, compared to people on very low doses.

People who had just started taking opioids weren't at especially high risk of injury, Gomes told Reuters Health. The period just after doses are increased may be more critical, she said.

"As you are changing your dose of opioids and increasing your dose of opioids, you may want to see how that impacts your alertness, and if you feel that you are not as sharp as you usually are you may want to avoid driving until you feel accustomed to that dose," she advised.

The researchers cautioned that they didn't know how pain patients were using their opioids - whether they took them correctly or some sold or bought the drugs illegally, for example.

Pain expert Dr. Angela Mailis-Gagnon from Toronto Western Hospital, who wasn't involved in the new study, said the research had a number of limitations - in part due to the findings being from an administrative database without some important clinical details.

For example, she said, the study didn't take into account that some opioids come in both immediate- and slow-release forms. Immediate-release forms hit patients much harder, she said, and may make them more unstable.

In some cases, the research team also didn't account for other, non-opioid medications the people were taking that could affect attention during driving, she added.

Gomes said trouble with driving isn't one of the classic side effects that doctors warn patients about with opioids - but maybe it should go on that list.

"There are millions of people in the U.S. who are taking opioids at these doses, and so this isn't a small issue," Gomes said.

Mailis-Gagnon, however, said she didn't want the new findings to overly worry patients who take low- or moderate-dose opioids as prescribed and really need the drugs.

"There is no question that opioids affect driving," she told Reuters Health - but maybe not at all doses, and for all patients. "It gives the wrong message out there, and that is what concerns me greatly."

SOURCE: bit.ly/WHPOHo JAMA Internal Medicine, online January 14, 2013.

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