Scicurious has a clear explanation of this addiction model on Neurotopia v 2.0:

So what is the opponent-process theory? The opponent-process theory (hereafter called the OP Theory) is one of the current theories we are using to understand addiction. Because, to be honest, we don’t really understand it. Oh sure, we know about initial rewarding effects, we know about withdrawal, we know about tolerance. But do we really KNOW what it is that makes people walk away from their families and homes and jobs and sell themselves for their next hit? A next hit that, oftentimes, they HATE and need at the same time? …nope. Still working on that.

But one of the theories out there to explain drug addiction and how it may work is the OP Theory.

There are a few nice diagrams in this post to help you understand it, but I’m having a hard time figuring out where the HP is supposed to go and what color the character defects are. I guess that’s why the author has to concede that:

There’s only one issues with the OP Theory: it’s probably wrong. But it’s wrong in the way that most of the theories we have about understanding the brain, and particularly about understanding things like motivation and reward, are wrong. It may not be entirely wrong, it may just be incomplete.