Doctors give no documented reason for prescribing opioids in 29 percent of cases, according to a study published this week in Annals of Internal Medicine.

The study analyzed opioid prescriptions written during 809 million outpatients visits over 10 years. Two-thirds of the prescriptions were intended to treat non-cancer pain, and 5 percent were written for cancer-related pain. The rest were written without a record of pain symptoms or a pain-related condition, CNN reports.

“If a doctor does not document a medical reason for prescribing an opioid, it could mean that the prescription is not clinically appropriate,” said lead author Dr. Tisamarie Sherry. “But it could also mean that the doctor simply missed recording the medical justification for an opioid, perhaps due to time constraints, clinic workflows or complicated documentation systems.”