> That (logically) means that your body either retains something it should have gotten rid of or the opposite, meaning your are changing the composition of your body.
A change in the chemical composition of urine strongly suggests a change in the body, but doesn't "logically" prove it. In the simplest case you could imagine consuming some kind of dye which is simply excreted and not metabolized, which could change the appearance of your urine, kind of like in
(where the urine change itself mainly reflects what the body did not absorb).
Another possibility is that the body breaks down the drug and excretes all of the reaction products. In principle, that could occur without consuming any other substance, just by applying energy to cause a reaction to occur, or by providing a catalyst.
Also, some chemical changes in the body are beneficial (though the typical random change is most likely not).
A change in the chemical composition of urine strongly suggests a change in the body, but doesn't "logically" prove it. In the simplest case you could imagine consuming some kind of dye which is simply excreted and not metabolized, which could change the appearance of your urine, kind of like in
https://en.wikipedia.org/wiki/Beeturia
(where the urine change itself mainly reflects what the body did not absorb).
Another possibility is that the body breaks down the drug and excretes all of the reaction products. In principle, that could occur without consuming any other substance, just by applying energy to cause a reaction to occur, or by providing a catalyst.
Also, some chemical changes in the body are beneficial (though the typical random change is most likely not).