by colonic bacteria to urobilinogen, which is then further oxidized to urobilin and stercobilin. Urobilin, stercobilin and their degradation products give feces its brown color. However, just like bile, some of the urobilinogen reabsorbed is resecreted in the bile which is also part of enterohepatic circulation. The rest of the reabsorbed urobilinogen is excreted in the urine where it is converted to an oxidized form, urobilin, which gives urine its characteristic yellow color. Drugs Chloramphenicol, aspirin, paracetamol, diazepam, lorazepam, morphine, metronidazole. Not only drugs but endogenous substrates like bilirubin, steroidal hormones and thyroxine utilize this pathway. Enterohepatic circulation of drugs describes the process by which drugs are conjugated to glucuronic acid in the liver, excreted into bile, metabolized back into the free drug by intestinal bacteria, and the drug is then reabsorbed into plasma. For many drugs that undergo this process, lower doses of drugs can be therapeutically effective because elimination is reduced by the 'recycling' of the drug. But for a small number of drugs that are very toxic to the intestine (e.g. irinotecan), these molecules which would not otherwise be very toxic can become so because of this process, and therefore inhibition of this step can be protective. For the majority of drugs which undergo enterohepatic circulation that are not toxic to the intestine, inhibition of this process leads to a reduction of the levels of drug and reduced therapeutic effect. For example, antibiotics that kill gut bacteria often reduce enterohepatic drug circulation and this requires a temporary increase of the drug's dose until the antibiotic use is discontinued and the gut repopulates with bacteria. This effect of antibiotics on enterohepatic circulation of other drugs is one of several types of drug interactions. Pharmacokinetic Models of enterohepatic circulation Pharmacokinetic models of the enterohepatic circulation process has been summarized in a recent article. See also Enteroenteric circulation References Hepatology Angiology Digestive system