P-glycoprotein (P-gp) is a gatekeeper at the gut lumen and the blood-brain barrier. P-gp pumps P-gp substrates out of the brain—“Pumpers gonna pump”.
An example of a relevant P-gp interaction involves the OTC opioid antidiarrheal loperamide (Imodium). Loperamide does not cause central opioid effects under normal circumstances. If the individual takes a potent P-gp inhibitor, megadose loperamide can stay in the brain long enough to cause euphoria. The P-gp inhibitor typically used the achieve this recreational effect is omeprazole (Prilosec).