Adults with a mental illness receive more than 50 percent of the 115 million opioid prescriptions in the United States annually, according to a study released Monday.

People with mental health disorders represent 16 percent of the U.S. population.

“Because patients with mental health disorders are a vulnerable population, (they’re) probably more likely to develop addiction and abuse,” said Dr. Brian Sites, an anesthesiologist at Dartmouth-Hitchcock Medical Center in New Hampshire and one of the study’s researchers.

The study, published in the Journal of the American Board of Family Medicine, found that nearly 19 percent of Americans with a mental health illness use prescription opioids, while the same is true for only 5 percent of those without a mental health condition.

According to the federal Centers for Disease Control and Prevention, the number of opioid sales in the U.S. quadrupled from 1999 to 2015, yet the amount of pain adults experience remained the same. In addition to that, more than 183,000 people died from overdoses related to prescription opioid use during this time.

With no objective scale for measuring pain, doctors are hampered in treating patients with chronic discomfort.

“Since (pain is) a subjective phenomenon, it’s very difficult to measure those things and to treat because some patients (report) 10-out-of-10 pain forever,” Sites said.

Sites said alternatives to opioids could include cognitive behavioral therapy, acupuncture, meditation techniques and physical therapy.