I think the guy was a private investigator. He has a sex change girlfriend, takes pills to get different boosts of sorts and has pills to give them a softer let down, I remember triangle and star shaped pills I think? I was thinking the triangle pills were blue.
Most also have a jack in port in their head that lets them gain new info and know things. This might be wrong, but I think he was one who had avoided getting a brain port, but in the course of the book he got one. I do remember that at the end he gets in a fight and plugs a black chip into his port and goes feral. He doesn't remember anything, but they tell him it looks like an animal had killed everyone there, everyone he knew was disgusted with it and left him basically under the power of the manipulator who had gotten him the black chip.
I found out later that this is part of a series of books, not sure how long. I read it 15 years ago or so, sorry if my details are lacking.