While some studies did not find a correlation between P-gp levels and fentanyl transport [4,7], others showed that P-gp inhibition increased the passage of fentanyl through the BBB towards the CNS, with consequent enhancement of its antinociceptive and side effects (e.g., dose-dependent respiratory depression and prolonged duration of the loss of righting reflex). Here, PGP is linked to respiratory depression.