Many patients who have undergone knee or hip replacement surgery are still taking prescription opioid painkillers up to six months after the operation, a new study finds. Some patients continue to use potentially addictive pain medications even though their hip or knee pain has improved, the findings suggest.

The study also suggests persistent opioid use after knee or hip replacement surgery may be more common than previously reported, the researchers said.

Continued opioid use after joint replacement surgery is not necessarily related to pain in the affected joint, the researchers said. “We hypothesize that the reasons patients continue to use opioids may be due to pain in other areas, self-medicating affective distress, and therapeutic opioid dependence,” they wrote in the journal Pain.

Researchers at the University of Michigan studied opioid use in 574 patients undergoing knee or hip replacement surgery, Science Daily reports. They followed up with patients one, three and six months after the procedure. They found about 30 percent of patients were taking opioids before their surgery. Of this group, 53 percent of knee replacement patients and 35 percent of hip replacement patients were still taking opioids six months after the operation.

Patients who had not taken opioids before surgery were less likely to persistently take opioids afterwards. About 8 percent of knee replacement patients in this group, and 4 percent of hip replacement patients, continued to take opioids after six months.

Patients most likely to take opioids long-term after their surgery were those who took high doses of opioids before the operation. Patients who took the highest doses of opioids before the procedure had a predicted probability of persistent opioid use of 80 percent six months after surgery.

In patients who had not previously taken opioids, those with more pain on the day of surgery – both in the affected joint and overall body pain – were more likely to persistently use opioids six months after the surgery.