Furanocoumarins (see section above) irreversibly inhibit a metabolizing enzyme cytochrome P450 3A4 (CYP3A4). CYP3A4 is a metabolizing enzyme for almost 50% of drugs, and is found in the liver and small intestinal epithelial cells. As a result, many drugs are impacted by consumption of citrus juice. When the metabolizing enzyme is inhibited, less of the drug will be metabolized by it in the epithelial cells. A decrease in drug metabolism means more of the original form of the drug could pass unchanged to systemic blood circulation. An unexpected high dose of the drug in the blood could lead to fatal drug toxicity.