A new study from the Johns Hopkins Bloomberg School of Public Health found that opioid prescription use increased significantly between 1999 and 2014, and that much of that increase stemmed from patients who’d been taking their medication for 90 days or longer.

Long-term use, which is associated with greater risk for addiction and overdose, increased threefold during the study’s time frame. In 1999-2000, less than half of the people who were taking prescription opioids were taking them for 90 days or more. By 2013-2014, more than 70 percent were taking opioid medication on a long-term basis.

The findings come as the U.S. grapples with a worsening opioid epidemic that on average is killing nearly 100 people a day, some from prescription opioids and others from illegal forms, primarily heroin. Last month, the Trump administration declared the opioid epidemic a public health emergency, a step that will allow the government to dispense additional federal funds for treatment.

The study, published online in the journal Pharmacoepidemiology and Drug Safety, draws from survey data gathered by the National Health and Nutritional Examination Survey, which the National Center for Health Statistics has conducted every two years since 1999-2000. Prescription opioid use, the paper found, rose from 4.1 percent of U.S. adults in 1999-2000 to 6.8 percent in 2013-2014, an increase of 60 percent. Long-term prescription opioid use, defined as 90 days or more, increased from 1.8 percent in 1999-2000 to 5.4 percent in 2013-2014.

“What’s especially concerning is the jump in long-term prescription opioid use, since it’s linked to increased risks for all sorts of problems, including addiction and overdoses,” says study author Ramin Mojtabai, MD, PhD, MPH, a professor in the Department of Mental Health at the Bloomberg School. “The study also found that long-term use was associated with heroin use as well as the concurrent use of benzodiazepines, a class of widely prescribed drugs that affect the central nervous system,” he said.

This is one of the paper’s more worrisome findings, Mojtabai notes, since combining opioids and benzodiazepines significantly increases the risk of overdose, even if the patient is taking a moderate dosage of opioid medication. Combining these drugs can also cause respiratory suppression, he said.

For the paper, Mojtabai examined eight consecutive biannual surveys, each of which included over 5,000 adults living throughout the U.S. Interviews were conducted via computer in participants’ homes. Participants were asked to identify prescription medication they’d taken in the past 30 days, and for what length of time. A total of 47,356 adults participated in the eight surveys, and the response rate ranged from 71 percent to 84 percent. If participants were taking more than one opioid medication, the study logged the duration for the longest-used medication.

Opioid medication use overall and long-term use were more common among participants on Medicaid and Medicare versus private insurance.

Despite the upward trend, there is scant evidence supporting benefits of longer-term prescription opioid use, Mojtabai says, with no randomized clinical trials that support their extended use, given the risks.

Prescription opioids were originally designed for shorter-term use, which involves fewer patient risks. Many patients who take opioid medication for weeks or months develop a tolerance that over time requires higher dosages for the medication to relieve pain. As a result, patients will take more medication to reduce their pain, setting them on the path to possible addiction. While there is no clear delineation as to when addiction kicks in, longer-term use is thought to be a risk factor.