After intrathecal administration, opioid drug disposition depends on the lipid solubility of the individual drug. Because of its hydrophilic nature, CSF concentrations of morphine decline more slowly than similar doses of lipophilic drugs. This accounts for more rostral spread, greater dermatomal analgesia, and longer duration of action when compared to highly lipophilic opioids like fentanyl and sufentanil. When used for post-cesarean analgesia, intrathecal morphine has a duration of action between 14-36 hours with wide variation between individual patients. While hydromorphone is similar chemically to morphine, it is more lipid soluble. This decreases its spread within the intrathecal space and enhances its penetration into the dorsal horn of the spinal cord where interactions with opioid receptors occur. These differences between the two medications may influence their duration of action. Theoretically, this would reduce the duration of action of intrathecal hydromorphone when compared with intrathecal morphine. Retrospective studies have shown that the analgesic benefit for intrathecal hydromorphone appears to extend at least 12 hours after cesarean delivery and may extend up to 24 hours.