Large observational study suggests Tramadol is associated with increased risk of adverse outcomes compared to Codeine.

Association of Tramadol vs Codeine Prescription Dispensation With Mortality and Other Adverse Clinical Outcomes – JAMA (free for a limited period)

Editorial: Tramadol, Codeine, and Risk of Adverse Outcomes – JAMA (free for a limited period)

Related: Tramadol: Where do we go from here? – “Tramadol’s unpredictability, unique potential harms, and lack of advantage over other opioids or NSAIDs are now well recognized”.