People who use higher doses of narcotic painkillers to manage pain are at increased risk of depression, a new study suggests.

The study included 355 patients who reported low back pain at an initial doctor’s visit, and at one-year and two-year follow-up visits, HealthDay reports.

The findings are published in the journal Pain. The researchers have found most of the risk of depression is due to the duration of narcotic painkiller use, not the dose, according to lead researcher Jeffrey Scherrer.

“A strong potential explanation of our finding that increasing opioid dose increases risk of depression could be that the patients who increase dose were the longer-using patients,” Scherrer noted. “This is logical as longer use is associated with tolerance and a need to increase opioids to achieve pain relief.”

Learning more about the link between these painkillers and depression, along with what dosage might put patients at higher risk, “may inform prescribing and pain management” by doctors, the researchers wrote.

“We hope to find risk factors such as opioid misuse that could be in the pathway from chronic opioid use to new onset depression,” Scherrer said in a news release. “This would expand the targets for intervention to limit the risk of depression in patients who need long-term opioid therapy.”