Lyric discussion by CheeseLord 

Cover art for Down in It lyrics by Nine Inch Nails

Pretty obvious its about heroin.

@CheeseLord that's the impression I got from it

@CheeseLord That’s always been my opinion. “Just then a tiny little dot caught my eye.” And “I watched it way too long. It was pulling me down.” I took it to mean his track mark. I got hooked on it for a few years and it is awful. He’s saying his life was great and now he’s in the shit because of the addiction.