Figure 1: Alterations in transmitter systems found in fibromyalgia patients in terms of increased or decreased activity and action of drugs used in fibromyalgia on these transmitters systems in terms of activity increase or decrease. *The positive effect of naltrexone, an opioid antagonist, on fibromyalgia symptoms is suggested to be mediated through disinhibiting effects on HPA axis activity, rather than by its effect on the opioid system (cf. [1618]).