Opioid switching to methadone: a pharmacoepidemiological study from a national prescription database.

BACKGROUND Opioid switching to methadone is reported frequently to improve pain control in patients with an unacceptable balance between pain control and side effects during treatment with first line opioids, but carries a risk of drug accumulation and respiratory depression. To justify this risk it is required that less risky treatments are tried beforehand and that a sufficiently large proportion of patients experience a long-lasting improvement in pain control. RESEARCH QUESTIONS How large was the proportion of patients remaining on long term methadone treatment after a switch from a strong opioid to methadone? How long had the patients been treated with opioids before the switch to methadone? METHODS Longitudinal pharmacoepidemiological study from the complete national Norwegian Prescription Database. RESULTS One hundred and thirty (77%) cancer patients received more than one dispensed prescription of methadone. Forty-nine (40%) chronic non-malignant pain (CNMP) patients continued to have methadone prescriptions dispensed more than 6 months after the switch. Of 168 cancer patients, 48 (29%) had tried two strong opioids prior to the switch to methadone whereas 21 (12.5%) had tried three or more strong opioids. Similar numbers for 124 CNMP patients were 26 (21%) and 34 (27%), respectively. INTERPRETATION Opioid switching to methadone appears to provide a long lasting improvement in pain control in a significant proportion of patients. However, the study raises concerns that treatment options with less risk are not being exhausted prior to switching to methadone.