Illustration showing the hepatic portal vein system. The<unk>  (also known as<unk> -pass metabolism or presystemic metabolism) is a phenomenon of drug metabolism whereby the concentration of a drug, specifically when administered orally, 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<unk>  are imipramine, morphine, propranolol, buprenorphine, diazepam, midazolam, pethidine, tetrahydrocannabinol (THC), ethanol (drinking alcohol), cimetidine, lidocaine, and chlorpromazine. In contrast some drugs are enhanced in potency: for example, the effect of THC - the most widely studied active ingredient in cannabis - is enhanced by transformation of a significant portion into 11-hydroxy-THC, resulting in greater potency than the original THC.<unk> <unk>  metabolism may occur in the liver (for propranolol, lidocaine, clomethiazole, and NTG) 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<unk> <unk>  through the liver thus may greatly reduce the bioavailability of the drug. An example of a drug where<unk> <unk>  metabolism is a complication and disadvantage is the antiviral drug, Remdesivir. Remdesivir cannot be orally administered because the entire dose would be trapped in the liver with little reaching the systemic circulation and reaching organs and cells affected by, for example, SARS-CoV-2. For this reason, Remdesivir is administered by IV infusion, bypassing the portal vein. However, significant hepatic extraction still occurs because of second<unk>  metabolism, whereby a fraction of venous blood travels through the hepatic portal vein and hepatocytes. The four primary systems that affect the<unk>  of a drug are the enzymes of the gastrointestinal lumen, gut wall enzymes, bacterial enzymes, and hepatic enzymes. In drug design, drug candidates may have good druglikeness but fail on<unk> -pass metabolism because it is biochemically selective. Alternative routes of administration, such as insufflation, suppository, intravenous, intramuscular, inhalational aerosol, transdermal, or sublingual, avoid the<unk>  because they allow drugs to be absorbed directly into the systemic circulation. Drugs with high<unk>  typically 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<unk> <unk>  metabolism, frequently among several other factors. Oral bioavailability of many vulnerable drugs appears to be increased in patients with compromised liver function. Bioavailability is also increased if another drug competing for<unk> <unk>  metabolism enzymes is given concurrently (e.g., propranolol and chlorpromazine).
First pass effect
Partial volume 
Weekend effect
Therapeutic index
Rebound effect
Tissue selectivity
Gateway drug effect
Relative biological effectiveness