Eight healthy male volunteers underwent an acute precipitated narcotic drug withdrawal protocol as previously described [30], with or without ondansetron pretreatment using a randomized double-blinded placebo-controlled crossover study design on two separate occasions. Test sessions for each individual volunteer were separated by at least 7 days. Female volunteers were excluded from the study because of modulation of opioid response by menstruation cycles [31]. The human experimental protocol was approved by the Institutional Review Board (Stanford University, USA). Each volunteer provided written informed consent before study enrolment, and the study was registered in the clinicaltrials.- gov database (identifier NCT00661674).
