I believe some substances (medicines) go through, some are altered so they can be passed through to the urine and are then not the same as when they were taken and some do not significantly pass through.
Try doing Google searches on the drug name and urine to see if you can find anything. Otherwise you might be able to make a request to the drug company.
I think I read somewhere that the liver attaches enzymes to the substance which allows the kidneys to eliminate it in some cases. You must also think about the possible potency in urine which may be very small and also that it is going into your stomach with food, not straight into your bloodstream.