Although most drugs that are metabolised by the liver end up in the plasma, some drugs are excreted in bile. After being excreted via bile into the intestine, the drug or metabolite leaves the body with the faeces after hydrolysis in the intestine. However, the drug and/or its metabolite can also be taken up again and transported to the liver via the portal circulation. This reuptake from the intestine is called enterohepatic circulation. The effect of this phenomenon involves the formation of a reservoir of recirculating drugs (metabolized and unchanged) and prolongation of drug action.
Examples of drugs that are excreted extensively into bile include:
- Morphine (undergoes enterohepatic circulation)
- Ethynylestradiol (undergoes enterohepatic circulation)
- Ceftriaxone (undergoes biliary excretion)