Apparently, another factor may be involved in this response, such as genetic polymorphism as reported in an experiment with healthy subjects after a single opioid administration (Jensen et al., 2009) and in a group of cancer patients (Rakvåg et al., 2005), which showed that after a single opioid administration, a repeated painful stimulus produced a hyperalgesic effect, which was associated with the COMT val158met polymorphism. Here, COMT is linked to cancer.