Like other opioid drugs, pethidine has the potential to cause physical dependence or addiction. It may be more likely to be abused than other prescription opioids, perhaps because of its rapid onset of action. When compared with oxycodone, hydromorphone, and placebo, pethidine was consistently associated with more euphoria, difficulty concentrating, confusion, and impaired psychomotor and cognitive performance when administered to healthy volunteers. The especially severe side effects unique to pethidine among opioids—serotonin syndrome, seizures, delirium, dysphoria, tremor—are primarily or entirely due to the action of its metabolite, norpethidine.