Liver injury is a possible consequence of ingestion of any xenobiotic, including industrial toxins, pharmacologic agents, and complementary and alternative medications (CAMs). Among patients with acute liver failure, drug-induced liver injury is the most common cause, and evidence for hepatotoxicity detected during clinical trials for drug development is the most common reason for failure of compounds to reach approval status. Drug-induced liver injury requires careful history taking to identify unrecognized exposure to chemicals used in work or at home, drugs taken by prescription or bought over the counter, and herbal or dietary supplement medicines. Hepatotoxic drugs can injure the hepatocyte directly, e.g., via a free-radical or metabolic intermediate that causes peroxidation of membrane lipids and that results in liver cell injury. Alternatively, a drug or its metabolite may activate components of the innate or adaptive immune system, stimulate apoptotic pathways, or initiate damage to bile excretory pathways (Fig. 361-1). Interference with bile canalicular pumps can allow endogenous bile acids, which can injure the liver, to accumulate. Such secondary injury, in turn, may lead to necrosis of hepatocytes; injure bile ducts, producing cholestasis; or block pathways of lipid movement, inhibit protein synthesis, or impair mitochondrial oxidation of fatty acids, resulting in lactic acidosis and intracellular triglyceride accumulation (expressed histologically as microvesicular steatosis). In other instances, drug metabolites sensitize hepatocytes to toxic cytokines. The differences observed between susceptible and nonsusceptible drug recipients may be attributable to HLA haplotypes that determine binding of drug-related haptens on the cell surface as well as to polymorphisms in elaboration of competing, protective cytokines, as has been suggested for acetaminophen hepatotoxicity (see below). Immune mechanisms may include cytotoxic lymphocytes or antibody-mediated cellular cytotoxicity. In addition, a role has been shown for activation of nuclear transporters, such as the constitutive androstane receptor (CAR) or, more recently, the pregnane X receptor (PXR), in the induction of drug hepatotoxicity.