It turns out that many medications we consume do not get fully processed in our bodies and they are often excreted via urine unchanged (our bodies simply do not use them all).
Modern sewage treatment plants do not use any processes to remove or neutralize these medications - most of these medical compounds where invented after the modern sewage treatment mechanisms we use today were put into large scale use - so they simply pass on through and into the environment.
Virtually all sewage plants emit effluent into waterways - either a river, lake or ocean - so that's were these compounds end it.
An individual can excrete only a tiny amount which would normally be inconsequential - but with what I'd estimate to be at least a half a billion women peeing out birth control compounds each day the micrograms of medication start to add up (from this I estimated a half billion women - 20% of the women on all continents save Africa and Antartica use these products).
It would seem today that indeed fish exposed to things like anxiety medications in fact have measurably altered behavior (see this from the Washington post).
Birth control medications enter the water systems via urine. Other compounds, such as anxiety medications, enter these systems via disposal via from unglamorous means such as being "flushed down the toilet."
Like cell phones used by teens to "sext" it would seem the consequences of these modern medications are only now coming to light.
Since these compounds have been in our water systems for decades even experiments described in the article by the Washington Post are suspect because no one knows how long the fish being used in the experiments described have been exposed to these drugs.
Little wonder the big pharma companies want nothing to do with the environmental consequences of these medications.