First-pass metabolism

Breakdown of a swallowed drug by the liver before it reaches the rest of the body.

Anything absorbed from the gut travels to the liver before it reaches the general circulation. The liver may chemically alter a substantial share of the dose on this first pass, so only the remainder goes on to have an effect. Drugs with heavy first-pass metabolism need much larger oral doses than injected ones, and some cannot be given orally at all. It is also why some medications are formulated to dissolve under the tongue, which routes them into the bloodstream while bypassing the liver.

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