What is the cytochrome P450 system and why does it matter for drug interactions?

The cytochrome P450, or CYP450, system is a family of liver enzymes responsible for metabolizing approximately 75 percent of all medications. The most clinically important isoenzymes are CYP3A4, CYP2D6, CYP2C9, CYP2C19, and CYP1A2. Some drugs inhibit these enzymes, slowing the metabolism of other drugs and increasing their blood levels and toxicity risk. Other drugs induce these enzymes, speeding metabolism and reducing drug effectiveness. For example, grapefruit juice inhibits CYP3A4, increasing levels of statins, calcium channel blockers, and many other drugs.