Perhaps it’s not surprising, but people who take high doses of opioid painkillers even for legitimate medical reasons are at risk of overdosing, new research shows.

The findings add to the ongoing controversy over this class of potentially addictive drugs, which includes morphine and oxycodone (Oxycontin). When taken in excess, they slow the breathing muscles, and users suffocate.

However, the results do not mean patients should stop taking prescribed medication, experts say.

For the study, published in the Annals of Internal Medicine, researchers followed nearly 10,000 adults who had received at least three opioid prescriptions within 90 days to treat chronic pain such as backache. Of these, 51 experienced at least one overdose, and six died as a result.

The researchers also found that the higher the painkiller dose, the more likely the patients were to overdose. Among those receiving the equivalent of less than 20 milligrams of morphine per day, one in 500 overdosed per year. That compared to about one in 60 among those who received the equivalent of 100 milligrams or more.

Several million Americans now use opioids to relieve disabling chronic pain, and so even relatively small overdose rates could amount to thousands of overdoses every year.

“Opioids carry risks and prescribing them and using them long-term is something that should be done with caution,” said Michael Von Korff, who led the new research.

“I’m not interested in scaring people,” added von Korff of Group Health Cooperative, a nonprofit health-care system based in Seattle, Wash. “If someone is taking a couple Percocet (an opioid painkiller) a day and they are doing fine…I don’t think anybody is particularly concerned about that.”

Yet as prescriptions have skyrocketed over the past two decades, so have fatal overdoses, which now number in the thousands each year.

“As a society, we have underestimated the possible risks from the dramatic increase in use of opioids,” said Dr. Leonard Paulozzi of the U.S. Centers for Disease Control and Prevention, who was not involved in the new study.

People who overdose tend to be drug abusers without a prescription, but until now the risk in people with legitimate prescriptions has been unclear. In the study, overdoses were particularly common among people who had a history of depression or substance abuse.

“The study is really illuminating and insightful, because it’s calling into question the notion that as long as you have legitimate pain, you really don’t have to worry about problems with substance-use disorders,” said Dr. Douglas Gourlay, an anesthesiologist and addiction expert at Mount Sinai Hospital in Toronto, Canada, who was not involved in the study.

However, the findings should not deter doctors from prescribing opioids, he said. The drugs are used mostly as second-line treatment for patients who have failed to get relieve with other painkillers, such as ibuprofen and aspirin.

Instead, doctors ought to pay more attention to assessing patients for abuse potential before they begin treatment, Gourlay said. Several guidelines already recommend careful patient assessment and monitoring, but it’s unclear how widely busy primary care physicians adhere to this practice.

As for patients, Von Korff said, “if you’re using opioids long-term, you should never use more than your doctor prescribed. And if you’re having problems-you’re falling asleep, you feel over-sedated-you should tell your doctor.”

SOURCE: Annals of Internal Medicine, January 19, 2010.