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The administration of neuraxial (epidural and intrathecal) opioids has been shown to induce pruritus in several species, including horses, sheep, dogs, rats and humans. This side effect appears to be dose‐dependent. The exact mechanism/s of opioid‐induced pruritus is unclear, but it is likely that more than one mechanism is involved, including the mu and kappa opioid receptors, modulation of serotonergic pathways, prostaglandins release [91] or the activation of 5‐HT3 receptors [92]. In horses, the few reported cases of pruritus were associated with epidural administration of morphine (preservative‐free) through an epidural catheter alone [93, 94] or in combination with detomidine [95]; therefore, it could not be totally ruled out that pruritus was caused by local irritation due to the catheter itself or by detomidine or its preservative.

In humans, parturients are the most susceptible to this complication, which may be due to the interaction of estrogen with opioid receptors [91].

Co‐administration of epinephrine (adrenaline) may increase the severity or duration of the pruritus as it decreases the vascular uptake of the opioid from the epidural space, increasing its concentration, although the evidence is conflicting [91].

In horses, there are only 3 case reports (4 horses) of pruritus associated with epidural administration of morphine through an epidural catheter [93–95]. The incidence of this complication in horses is unknown but in a retrospective study on the outcomes of epidural catheterization in 43 cases, no horses developed pruritus [87]. In humans, the reported incidence of pruritus after neuraxial opioids is between 30% and 100% [91].