Herbal medicines have been frequently used for thousands of years in the East in the hope of treatment of some diseases. Patients who self-medicate with herbs for preventive and the- rapeutic purposes may assume that these products are safe because they are natural but some products cause adverse effects or have the potential to interact with prescription medi- cations despite their mechanisms of action being generally unknown, the lack of evidence of efficacy, and inadequate toxi- cological data.There are some evidence that herbs such as ginkgo biloba, ginseng, kava, garlic, echinacea,valerian, st. john s worth have been reported to interact with drugs leading to clinically relevant adverse drug reactions. Herbal products contain several chemicals that are metabolized by phase 1 and phase 2 pathways and also serve as substrates for certain transporters. P-glycoprotein and cytochrome P450 3A4 toget- her constitute a highly efficient barrier for many orally absorbed drugs. Induction and inhibition of drug metabolizing enzymes and transporters by herbal component has been documented in several in vitro studies. Due to their interaction with these enzymes and transporters there is a potential for alteration in the activity of drug metabolizing enzymes and transporters the presence of herbal components Available literature studies, clinical reports and in vitro studies indicate that many drugs and active herbal constituents are substrates for both P-glycoprotein and cytochrome P450 3A4.