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After a drug is swallowed, it is absorbed by the digestive system and enters the hepatic portal system. It is carried through the portal vein into the liver before it reaches the rest of the body. The liver metabolizes many drugs, sometimes to such an extent that only a small amount of active drug emerges from the liver to the rest of the circulatory system. This first pass through the liver thus greatly reduces the bioavailability of the drug. Alternative routes of administration like intravenous, intramuscular, and sublingual avoid the first-pass effect because they allow drugs to be absorbed directly into the systemic circulation.
- First dose effect, a similarly-named but basically unrelated topic
- ADME, an acronym in pharmacokinetics and pharmacology standing for absorption, distribution, metabolism, and excretion
- Partition coefficient
- Biopharmaceutics Classification System
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