A new study shows that poor mental health correlates with an increased risk of long-term opioid use, suggesting that people with underlying mental health issues may be more prone to opioid addiction.

The study, published in the Pain journal, looked at data on prescriptions and health history for 10.3 million people from 2004 to 2013. It found that while 1.7% of people who were prescribed opioids were still using them six months later, the rate was higher for patients who had used psychoactive drugs or had a history of mental health issues. 

The study also found that among people who took medication for attention-deficit hyperactivity disorder (ADHD), the risk of long-term use was 1.5 times higher. And people with a history of opioid misuse were nine times more likely to be taking the drugs six months after their initial prescription. 

“We found that pre-existing psychiatric and behavioral conditions and psychoactive medications were associated with subsequent claims for prescription opioids," wrote study author Patrick D. Quinn, PhD, of Indiana University, Bloomington. 

The researchers suggest that doctors must consider a patient’s whole medical history before choosing to prescribe opioids. "Our findings support the ideas that clinical practice has deviated from the 'careful selection' under which most clinical trials are conducted and that thorough mental health assessment and intervention should be considered in conjunction with the use of long-term opioid therapy,” they wrote. 

The study pointed out that some of the negative side effects of opioid use—like depression or substance use disorder—are actually predictors for long-term use.

In January 2016, another research study linked long-term opioid use with depression. The study found that a significant number of long-term opioid users develop depression, and that the longer they are on the medications the more likely they are to become depressed. Of those who used opioids for longer than three months, 14.4% developed depression. This held true even when accounting for the association between pain and depression. 

“We really did rigorous control for pain, and we feel strongly that these results are independent of the known contribution of pain to depression,” said researcher Jeffrey Scherrer, an associate professor of family and community medicine at Saint Louis University in Missouri.

The reason for the connection to depression is still not understood. While there are some biological factors, there also seems to be a connection between mental health and opioid misuse, something that the current study highlights.