What is "o-desmethyltramadol"

O-Desmethyltramadol (O-DSMT) is an opioid analgesic and the main active metabolite of tramadol. (+)- O-DSMT is the most important metabolite of tramadol produced in the liver after tramadol is consumed. Tramadol is demethylated by the liver enzyme CYP2D6 in the same way as codeine, and so similarly to the variation in effects seen with codeine, individuals who have a less active form of CYP2D6 ("poor metabolisers") will tend to get reduced analgesic effects from tramadol. This also results in a ceiling effect (dependent on CYP2D6 availability) which limits tramadol's range of therapeutic benefits to the treatment of moderate pain.