10-[3-(4-Methyl-1-piperazinyl)propyl]-10H-phenothiazine is a potent inhibitor of the human p-glycoprotein (Pgp) transporter. It has been shown to inhibit the activity of PGP in vitro and in vivo, as assessed by flow assay, particle size, and titration calorimetry. 10-[3-(4-Methyl-1-piperazinyl)propyl]-10H-phenothiazine also inhibits the activity of the α7 nicotinic acetylcholine receptor. This drug may be useful for treating infectious diseases such as HIV/AIDS, hepatitis C, and malaria. 10-[3-(4-Methyl-1-piperazinyl)propyl]-10H-phenothiazine binds to crystalline cellulose and causes drug interactions with drugs that are substrates for other transporters such as p -glycoprotein.