The first pass effect (also known as first-pass metabolism or presystemic metabolism) is a phenomenon of drug metabolism whereby the concentration of a drug is greatly reduced before it reaches the systemic circulation. It is the fraction of drug lost during the process of absorption which is generally related to the liver and gut wall. Notable drugs that experience a significant first-pass effect are imipramine, morphine, propranolol, buprenorphine, diazepam, midazolam, pethidine, cannabis, cimetidine, lidocaine, and nitroglycerin. In contrast some drugs are enhanced in potency: for example, the effect of the most commonly considered active ingredient in cannabis, THC, is enhanced by transformation of a significant portion into 11-hydroxy-THC that more readily crosses the blood brain barrier and thus achieves greater potency than the original THC. [Cannabis edible]
First pass metabolism may occur in the liver (for propranolol, lidocaine, chloromethiasole and GTN) or in the gut (for benzylpenicillin and insulin).
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 suppository, intravenous, intramuscular, inhalational aerosol, transdermal and sublingual avoid the first-pass effect because they allow drugs to be absorbed directly into the systemic circulation.
Drugs with high first pass effect have a considerably higher oral dose than sublingual or parenteral dose. There is marked individual variation in the oral dose due to differences in the extent of first pass metabolism. Oral bioavailability is apparently increased in patients with severe liver diseases like cirrhosis. It is also increased if another drug competing with it in first pass metabolism is given concurrently. E.g. propranolol and chlorpromazine.[unreliable medical source?]
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