Like a toll plaza that slows and filters traffic before it reaches the highway, the body can reduce a drug's strength before it ever enters full circulation. First-pass metabolism is the process in which a substance absorbed through the stomach or intestines travels first to the liver, where enzymes break down part of it. Because of that early filtering step, less of the original drug reaches the bloodstream and brain than the person actually swallowed.

That matters because the same dose can affect people differently depending on how fast their liver works, what they ate, their age, other medications, and the form of the drug. A pill taken by mouth may produce weaker, slower, or less predictable effects than the same substance delivered another way. In DUI cases, that can become part of a dispute over impairment, drug concentration, timing, and whether a lab result matches observed behavior after a crash.

For injury claims, first-pass metabolism can also shape arguments about prescription use, side effects, and whether a driver or worker was actually impaired at the relevant moment. Experts may look at dosing history, liver processing, and toxicology results to decide whether a substance likely contributed to the incident or whether the timing points somewhere else.