The first aim of this study is the validation of monitoring seric concentrations of O-demethyl-tramadol and tramadol to make the ratio in order to detect CYP2D6 poor metabolizers in therapeutic situation, comparing the result with genotyping. The finding of a poor metabolizer status in a patient will make the choice of analgesic drugs easier, avoiding tramadol and codeine. The final objective of this research is to be able to determine the CYP2D6 phenotype in a patient treated by tramadol without a good analgesia. By a single take of blood and a rapid response, this method should be liked to improve pain management. Furthermore, CYP2D6 phenotyping is interesting for the patient because many other drugs depend on this way of metabolism.