What is norpropoxyphene?

Norpropoxyphene is a metabolite of propoxyphene, which is an opioid pain medication that was withdrawn from the market due to safety concerns. Norpropoxyphene has weak, partial agonist activity at the mu-opioid receptor, but it has a longer half-life than propoxyphene and can accumulate in the body with repeated dosing. This can increase the risk of toxicity, including respiratory depression, overdose, and death. Despite being a metabolite of a banned drug, some laboratories still test for norpropoxyphene in drug tests.