[Respiratory depression following medication change from tramadol to morphine].

Respiratory depression following change of medication from tramadol to morphine is described in two patients. Tramadol is metabolized by cytochromoxidase CYP2D6 to O-desmethyl-tramadol with opioid agonist activity. Of the western population 7% have a mutation of the gene responsible for CYP2D6, resulting in low enzyme activity. These persons will have little effect of tramadol. When tailoring analgesia, the lack of response to tramadol may be interpreted as a need for opioid dose increase. In such cases, excessive opioid doses may be prescribed resulting in opioid side effects.