fecal excretion

Not everything that goes into the gut comes back into the blood. Some of a drug simply passes through the intestine and leaves in the stool. Fecal excretion is the share of a drug that the body sheds this way, by the back door rather than through the kidney.

Two main sources feed it. First, an orally taken drug that is poorly absorbed never enters the bloodstream and is voided directly in the feces. Second, drug delivered to the gut in bile after biliary excretion may not be reabsorbed and is then lost in the stool. A smaller contribution comes from drug that diffuses or is secreted directly across the intestinal wall.

Fecal excretion matters most for drugs designed to act locally in the gut and for large or polar compounds that the kidney handles poorly. It is closely tied to enterohepatic circulation: if biliary drug is reabsorbed rather than excreted, it recycles and its fecal loss falls. Measuring drug recovery in both urine and feces is how researchers account for a drug's full elimination.