Limitless is the film version of Alan Glynn's book The Dark Fields. It was published in 2001, so it would've been written before or at most a year or two after modafinil got FDA approval, and long before modafinil was popularized by Wired, etc in the mid-2000s.

https://en.m.wikipedia.org/wiki/The_Dark_Fields

The book is really, really good. Much darker than the film, however. The ending is completely different, particularly the implications of the last paragraph.

If you're on a Dark Fields kick, Ted Chiang's 1991 short story Understand is worthwhile. Geekier and not as philosophically weighty, but way more fun.