"Earlier research has confirmed that fish absorb medicines because the rivers they live in are contaminated with traces of drugs that are not removed in sewage treatment plants. Much of the contamination comes from the unmetabolized residues of pharmaceuticals that people have taken and excreted; unused medications dumped down the drain also contribute to the problem."
Most people have no clue what to do with out of date meds - I used to dump them down the toilet - until I found out they get into the water supply. Meds have been found in well water, too.