Figure 2. Ionization, solubility, onset and duration of intrathecal opioids.

Hydrophilic opioids (i.e., morphine) have a slower onset and prolonged duration of action. Lipophilic opioids (i.e., fentanyl) demonstrate rapid onset and shorter analgesic duration.
Source: Ionization [27-29], Solubility [28,30], Onset and duration [31].