Docetaxel is one of the most widely used anticancer drugs. A major problem with docetaxel treatment, however, is the considerable interpatient variability in docetaxel exposure. Another disadvantage of the drug is that it has a very low oral bioavailability and can therefore only be administered i.v. The drug-metabolizing enzyme cytochrome P450 3A (CYP3A) and the drug transporter P-glycoprotein (P-gp; MDR1) are considered to be major determinants of docetaxel pharmacokinetics. It has been hypothesized that CYP3A and P-gp work synergistically in limiting the systemic exposure to many orally ingested drugs. However, it has been difficult to examine this interplay in vivo. We therefore generated mice lacking all CYP3A and P-gp genes. Although missing two primary detoxification systems, Cyp3a/Mdr1a/1b(-/-) mice are viable, fertile, and without spontaneous abnormalities. When orally challenged with docetaxel, a disproportionate (>70-fold) increase in systemic exposure was observed compared with the increases in single Cyp3a(-/-) (12-fold) or Mdr1a/1b(-/-) (3-fold) mice. Unexpectedly, although CYP3A and P-gp collaborated extremely efficiently in lowering docetaxel exposure, their individual efficacy was not dependent on activity of the other protein. On reflection, this absence of functional synergism makes biological sense, as synergism would conflict with a robust detoxification defense. Importantly, the disproportionate increase in docetaxel exposure in Cyp3a/Mdr1a/1b(-/-) mice resulted in dramatically altered and lethal toxicity, with severe intestinal lesions as a major cause of death. Simultaneous inhibition of CYP3A/P-gp might thus be a highly effective strategy to improve oral drug bioavailability but with serious risks when applied to drugs with narrow therapeutic windows.