In case anyone else is wondering after stumbling across this post, I finally after searching for months and years found the book title and series. It was a scholastic series called Explorer. The one I was searching for is called Destination Brain by Seth McEvoy.

Yes, very much like the Fantastic Voyage. I checked out those three but none of them seemed correct either. I really do believe it's from another series. I've checked Which Way and Twist-a-Plot, but neither seem correct either. 

I'm trying to find the title of a CYOA (or variant series) book, that centered around the main character being shrunk down in size and traveling through the human body, fighting blood cells, etc. It was very similar to the movie Inner Space, but definitely a choose your own adventure story. I remember reading it as a kid but can't for the life of me remember the title so I can find it again.

Any help would be appreciated!