enterohepatic recirculation

Enterohepatic recirculation is a recycling loop that keeps a drug in the body longer than expected. The liver dumps the drug into bile and out into the gut, but before it can be excreted in feces the gut hands it back, it is reabsorbed, and it returns to the liver — going round and round like luggage on an airport carousel.

The loop usually runs through conjugation. The liver glucuronidates a drug and secretes the glucuronide into bile, which empties into the intestine. There, bacterial enzymes (notably beta-glucuronidase) cleave off the sugar, regenerating the lipophilic parent drug, which is then reabsorbed across the gut wall and carried back to the liver via the portal vein to start again.

Clinically, this recycling prolongs a drug's residence and can produce a characteristic second bump in the plasma concentration curve some hours after dosing. It tends to lengthen the effective half-life and can complicate dosing. It is also why interrupting the loop — for example with bile-acid-binding resins or activated charcoal — can speed up elimination of a drug or toxin that undergoes substantial recirculation.