P-glycoprotein is a membrane transporter found in organs and tissues throughout the body that moves various compounds out of cells and is sometimes referred to as an efflux pump. P-glycoprotein can be found in the intestines, liver, blood-brain barrier, placenta, and kidneys, where it serves to defend the body from potentially toxic foreign substances. For example, in the intestines, blood-brain barrier, and placenta, p-glycoprotein prevents the absorption of foreign compounds into the blood, brain, and fetal circulation, respectively. In the liver and kidneys, p-glycoprotein can pump these compounds out of the organ to then be excreted from the body.^[1]

The function of p-glycoprotein can be affected by genetics, disease, and compounds found in food, medications, and supplements. Because some medications are p-glycoprotein substrates, their pharmacokinetics can be altered depending on the activity of p-glycoprotein. Drug interactions involving p-glycoprotein most often relate to its function in the intestines, where oral medications are absorbed into the body. By actively pumping these compounds out of intestinal cells and back into the intestine itself (the lumen) where they can be excreted, p-glycoprotein can reduce their bioavailability.^[1]