Opioid-induced adverse effects in the gastrointestinal system are primarily attributed to the activation of opioid receptors (μ-type, κ-type, δ-type) within the enteric nervous system, particularly in smooth muscle cells and in the terminals of sympathetic and sensory peripheral neurons in the gastrointestinal tract60. Consequences (symptoms) associated with opioid use include nausea, vomiting, constipation, abdominal distention, spasms and/or gastro-oesophageal reflux.