It is a fact that the human body does not totally absorb the total amount of the drug that we consume, so when we go to the bathroom we flush what is not consumed out of the body and it eventually goes into the ground; therefore, some eventually winds up in the water that is underground. By properly disposing of our prescription drugs that have expired or no longer needed in a proper disposal box at a Pharmacy/Drug Store we won't create a bigger problem of contaminating the potable (drinking) water supply.
If you live in a rural area you probably have a well and a septic tank and if in a hilly area the leaching fields are on a downgrade. If you're lucky, then the water will flow towards your down-slope neighbor's well from where he gets his water for drinking and cooking, but what if your neighbor is on the up slope, then the water from his leaching fields just might be running into your well. Generally, the trace amount that is excreted from the body should not pose a major problem, but if your neighbor disposes of a large quantity of prescription drugs it could possibly cause some contamination of the area water supply.
Everyone also has a varied supply of over-the-counter medicine for just-in-case. When these expire do you merely flush them down the toilet? Couldn't these also contaminate the water supply?
Some people may think that the sewers in the cities or those in a community sewer system can't pose a problem as they believe that the sewerage is properly disposed of, but is it? We all remember the poblem with the water supply in Flint a few years ago and what about this public water supply? “Residents of eight cities have been alerted that a brain-eating amoeba was found in a southeast Texas water supply, . . . .” Was this caused by a combination of legal and illegal drugs that were not disposed of properly? There are enough problems with the water without we humans deliberately adding to it. Dispose of your drugs properly.
There will never be any more or any less water on earth. What is here now will always be here. But, 97% of the water is in the oceans that we can not drink, 2% is frozen, and only 1% is potable water. Yes, we can desalinate ocean water like they do on ships and in dry countries bordering an ocean like Isreal, but that water is expensive and is only a drop in the bucket compared to that 1%. Let's all help to keep that 1% potable.