Chemoreceptor trigger zone, opioids activating

The opioid analgesics can activate the brainstem chemoreceptor trigger zone to produce nausea and vomiting. There may also be a vestibular component in this effect because ambulation seems to increase the incidence of nausea and vomiting. 

Nausea and emesis are common unpleasant side-effects of opioids (Campora et al., 1991 Aparasu et al., 1999). They are most intensively experienced at the beginning of the treatment. During chronic administration, tolerance may occur, which reduces the emetic sequelae. Nausea and emesis are induced via activation of chemoreceptors which are located in the trigger zone of the area postema of the formatio reticularis. The receptors are at the tissue surface and in contact with the circulating blood. Thus the emetic effect of opioids is not mediated centrally, i.e. after penetration of the blood-brain barrier, but rather peripherally via the amount of the compound, which is distributed in the circulating blood. 

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