First-pass metabolism is the process by which a drug taken orally is significantly metabolized in the liver and intestinal wall before it reaches the systemic circulation, thereby reducing the amount of active drug available in the bloodstream. After absorption from the gastrointestinal tract, the drug is transported via the portal vein to the liver, where enzymes (mainly cytochrome P450 systems) can chemically modify or inactivate a substantial portion of it. This effect is important in pharmacokinetics because it influences drug bioavailability, dosing requirements, and route of administration; drugs with high first-pass metabolism often require higher oral doses or are given through alternative routes such as sublingual, intravenous, or transdermal to achieve therapeutic levels.