What are the real effects that vicodin has on House? Are these effects only physical or do they go beyond that? Could they have somewhat changed his personality, even just a little bit?

I watched the episode "Need to Know" (the one after House kissed Stacy, when at the end of the episode he ends up driving her away) for the first time like 4 months ago (got hooked with House just this year) and I didn't give this whole House/Stacy situation a lot of thought, it was fun but I wasn't that interested in the ship.

But the plot with Stacy kind of surprised me, because while I was watching the first half of the first season I was pretty sure that House wasn't capable of sustaining a romantic relationship with anyone or at least not long enough for it to be that important. So yeah, I was surprised when Stacy came in the picture and we find out they were together for about 5 years, actually lived together and all. But, like I said, I didn't give it that much thought.

Now, I started re-watching the whole series after watching the season 5 finale and it gave me a whole new perspective. After watching this episode Need to Know again yesterday, it made me really wonder, why on earth would House turn away from the opportunity to have a relationship with this woman that he loves (or at least loved back then) and once had a relationship with?
Wilson's theory sounds quite right on to me, I'm pretty damn sure that he's right about why House rejected Stacy, or at least partially. But Wilson's theory overlooks a very important point, how come House was walking away from the relationship then, but embraced it so many years ago? Why did he say "yes" the first time, but says "no" the second time? What changed??

After watching the season 5 finale, I'm thinking that what changed is partly the leg pain, but an equally big of a part is the vicodin.
Non-vicodin House had plenty of issues and was screwed up alright, but was apparently capable of being in an actual relationship. Vicodin House has had no romantic relationships ever since the show started, he has come close to it a couple of times, has had at least two opportunities right there on his feet but he ended up rejecting them each time. Non-vicodin House wasn't exactly happy, but it didn't sound like he was that particularly miserable either. Vicodin House seems pretty damn miserable to me. Non-vicodin House seemed to be a bit on the crazy side. Vicodin House ends up going to the asylum. Non-vicodin House started a friendship with Wilson. Vicodin House almost terminated it.

So if we consider it all, the end of season 5 hardly seems like a coincidence, it has been a downslope for House to this point ever since he first started taking the vicodin, Kutner's death was just the final push.
In the episode "Detox", when House admits to Wilson that he has an addiction, they establish that he has changed a bit over the years, and Wilson asks him "And it's all the pain? Nothing's the pills?". Right then House could have claimed that it was all the leg pain, but he didn't, because deep down he knew the truth. After the end of season 5 I don't think he can deny it anymore, he is a changed man and a big deal of it is the vicodin.