p-gp is a glycoprotein that in humans is encoded by the "abcb1" gene. p-gp is a well-characterized abc-transporter (which transports a wide variety of substrates across extra- and intracellular membranes) of the mdr/tap subfamily. the normal excretion of xenobiotics back into the gut lumen by p-gp pharmacokinetically reduces the efficacy of some pharmaceutical drugs (which are said to be p-gp substrates). in addition, some cancer cells also express large amounts of p-gp, further amplifying that effect and rendering these cancers multidrug resistant. many drugs inhibit p-gp, typically incidentally rather than as their main mechanism of action; some foods do as well. any such substance can sometimes be called a p-gp inhibitor.