For any kind of drug (illegal or prescribed) it is never a good thing to put them in the toilet. Some compounds of these drugs are not eliminated by current treatments and are found in our streams and from later in our drinking water. With regard to the increase of nitrogen by drugs, some certainly have an effect on this level, depending on their chemical structure. Even when the drug is ingested or injected, your system evacuates part of it when you go to the bathroom, so whether it is thrown away in toilet or released by your system, any drug has an adverse effect on the environment in the end.
If you are asking about someone who found a couple of bottles of out of date bottles and flushed them, the realistic answer is that in a municipal wwtp that treats a million or more gallons per day, it will make no difference. That isn't the politically correct answer, but it is the realistic one.
If you are talking about a manufacturer, then the answer could be different. I am not privy to what a manufacturer drains. I would imagine that they have a hazardous waste protocol that must be followed, containerized and disposed of properly.