“Flowers for Algernon” was the title of the book. Algernon was the rodent on which the drug was first tested, and against which Charlie was measured when he started getting the drug. Algernon died, and it was a consequence of the drug. Then Charlie realized he, too, would die.
Yup. That's pretty much how I remember it. But I don't recall it as a book......as I remember it was in a collection (19XX's Best Science Fiction Stories or some such similar venue) But I seriously doubt that it was dying that got to Charlie. Even in his brilliant mode, he had to know that he (and we all) are going to die. It was the transition back to stupidity that was the horror.