Spices, herbal and black teas, and soybean products were analyzed for their capacity to inhibit in vitro metabolism of drug marker substrates by human cytochrome P-450 (CYP) isoforms. Inhibition of drug metabolism was determined using aliquots or infusions from these products in a fluorescence-detection assay. Aliquots and infusions of all natural product categories inhibited 3A4 metabolism to some extent. Of the 26 aliquots from teas and spices further tested with 2C9, 2C19 and 2D6, many demonstrated significant inhibitory activity on the metabolism mediated by these isoforms. Black teas and herbal tea mixtures were generally more inhibitory than single-entity herbal teas. Spices and single-entity herbal teas showed species-specific isoform inhibition with sage, thyme, cloves, St John's Wort and goldenseal having the highest activity against several isoforms. Seven soybean varieties tested, as well as daidzein and genistein isolated from soybean, were found to inhibit 3A4-mediated metabolism. Genistein was found to inhibit 3A7- but not 3A5-mediated metabolism of the marker substrate. Assessment of the in vitro CYP inhibition potential for these natural products has important implications for predicting the likelihood of natural product-drug interactions if these products are taken concomitantly.