People taking opioid painkillers for extended periods of time are at greater risk of problems if they have been prescribed more potent forms of these drugs, new research shows.

The study also found that people on long-term opioid therapy were more likely to visit the emergency room or to require medical care for overdose, withdrawal, intoxication or other alcohol- and drug-related reasons if they had been diagnosed with a substance abuse disorder previously, or if they reported having headaches or back pain.

Opioids are increasingly being used to treat chronic pain not related to cancer, Dr. Mark D. Sullivan of the University of Washington School of Medicine in Seattle and his colleagues note in their report in the Archives of Internal Medicine. Opioid abuse and deaths due to overdoses with prescription opiates also are on the rise.

To investigate whether certain factors might be related to adverse outcomes among patients prescribed opioids for non-cancer pain, Sullivan and his team looked at data on about 38,500 patients with commercial insurance and 10,000 who were covered through Arkansas' Medicaid program. All of them had used prescribed opioids for at least 90 days in a row for a six-month period between 2000 and 2005. For both groups, chronic back pain was the most common reason.

About 24 percent of the privately insured patients and 28 percent of the Medicaid patients visited the emergency department within a year of starting their long-term opioid use, the researchers found, while about two percent of the private insurance group and three percent of the Medicaid group required care for alcohol- or drug-related issues.

Having been previously diagnosed with a substance abuse disorder was the strongest predictor of whether a person on long-term opioid therapy would visit the emergency department or require alcohol- or drug-related care, the researchers found.

The type of drug a patient was prescribed also influenced risk. The Drug Enforcement Administration classifies drugs based on their medical benefit and potential for abuse.

Schedule I drugs -- such as heroin, for example -- have a high potential for abuse and no unique medical benefit. Most opioid drugs used to treat pain are Schedule II or Schedule III, with the Schedule II drugs such as morphine and oxycodone (also known as OxyContin) believed to have more potential for abuse than schedule III medications, which include combination drugs like Vicodin (hydrocodone and acetaminophen) or Tylenol 3 (acetaminophen with codeine).

People in the current study who were taking short-acting Schedule II opioids were more likely to visit the emergency department than those on non-Schedule II drugs, while people taking longer-acting Schedule II opiates were more likely to have alcohol- and drug-related encounters with the healthcare system.

Over the past few years, according to Sullivan, opioids have likely been overused for treating chronic pain unrelated to cancer. The researcher noted that in a decade's worth of research, he and his colleagues have found that the people who are at highest risk of suffering adverse outcomes from opioids -- for example younger people and those with a history of substance abuse -- are the most likely to be prescribed opiate drugs, more likely to receive more potent opiates, to be on higher doses, and to be on the drugs for a longer period of time.

The people who are the most likely to have trouble are the most likely to get the drug, Sullivan said.

While people 60 and older are much less likely to abuse opioids, Sullivan added, they are actually very worried about taking them. On the other hand, abuse is more likely among younger people, who are less wary about being prescribed opioids, he added.

Based on the findings, he and his colleagues conclude, avoiding prescribing the higher-risk drugs to higher-risk patients may improve the safety of long-term opioid therapy.

Medicine