Opioids are considered a "gold standard" in clinical practice for the treatment of postoperative pain. The spinal administration of an opioid drug does not guarantee selective action and segmental analgesia in the spine. Evidence from experimental studies in animals indicates that bioavailability in the spinal cord biophase is negatively correlated with liposolubility, and is higher for hydrophilic opioids, such as morphine, than lipophilic opioids, such as fentanyl, sufentanil and alfentanil. Epidural morphine sulphate has proven analgesic efficacy and superiority over systemically administered morphine for improving postoperative pain. However, pain relief after a single epidural injection of morphine could last less than 24 hours. Techniques used to administered and prolong opioid epidural analgesia, can be costly and inconvenient. Moreover, complications can arise from indwelling epidural catheterization, particularly in patients receiving anticoagulants. Clinical trials have shown that epidural morphine in the form of extended-release liposome injections (EREM) gives good analgesia for a period of 48 hours, with no need for epidural catheterisation. Intrathecal morphine produces intense analgesia for up to 24 hours with a single shot, and clinical recommendation is to choose the minimum effective dose and do not exceed 300 μg to prevent the delay respiratory depression.