Sometimes if you look up a pharmaceutical on wikipedia or elsewhere it will say that this drug is mainly excreted through the feces, not through urine or elsewhere.
But I thought the body is constantly processing and removing water and such from feces until they uh exit the body, so wouldn’t the drug in question just be reabsorbed?
I guess what I am asking is how does the body expel a drug in feces, and prevent itself from reabsorbing said drug until a bowel movement?
This means that the drug is mostly metabolized, or at least processed, by the liver, which excretes it in bile which in turn passes out through the feces.
There are also some pharmaceuticals which act in the gastrointestinal tract without being absorbed in the first place. I take a bile sequestrant, for instance, which isn’t supposed to be absorbed. An OTC medication which this applies for is simethicone - GasEx. That’s an anti-foaming agent which is intended to relieve bloating and gas pains by making the gas form larger bubbles which may be expelled more easily.
(And making TV ads for the stuff somewhat disingenous, it seems to me, by implying that it’s to keep you from farting. Essentially, it’s to make you fart (or belch) and get it over with.)
I used to think, reading the label, that simethicone made you not fart as much. In a way, it is true, but not in the way I thought. I thought it meant that it actually destroyed or otherwise diverted gas before it got all the way through the intestines so that fewer molecules would end up coming out. In reality, yes, it’s made to help you fart it all out now and not be tooting all afternoon.
Uptake of a drug requires the drug to either be taken up by specific transporters in the gut, or to be able to pass cell membranes. Substances can’t just “go along with the water” from the gastrointestinal tract to the lymph and blood.
Once in the blood, highly water soluble molecules are mostly excreted through the urine because the first step in this excretion is a filtration in the glomeruli of the kidney, where everything below a certain molecular size can pass. Most of the water, nutrients and other substances the body needs to retain are re-absorbed from the primary urine by specific transport mechanisms, all other components of the primary urine are excreted. Poorly water soluble substances bind to proteins in the blood and thereby form complexes that are too large to pass the kidney filter.
The liver oxidises such compounds to make them more water soluble and excretes them into the bile, which is released into the gastrointestinal tract. Because they were made more water soluble by the liver, they are no longer able to pass through cell membranes to be taken up into the blood again.