A selection of popular herbal medicinal products and food supplements were analysed for their potential to modulate the expression of the cytochrome P450 enzymes CYP1A2 and CYP3A4 and the transporter protein MDR1. A total of 31 products were analysed. Nine of the products have been approved by the Medical Products Agency (MPA) in Sweden and are marketed as herbal medicinal products. Twenty-two of the products have not been assessed by the MPA and are marketed as food supplements. LS180 cells were exposed to extracts from the different herbal products and real-time quantitative polymerase chain reaction, RT-QPCR, was subsequently used to analyse the relative mRNA levels of CYP1A2, CYP3A4 or MDR1 in treated and non-treated cells. Our results show that 17 of 31 products tested induced a two-fold expression or more for at least one of the genes analysed. Four products, of which a ginger-supplement was the most potent, induced all three genes.