This is for the Vault 95 companion quest that she gives you once you have enough approval. I specifically made a quicksave just outside the clean room to see if I could influence the outcome at all... and I could not. All four dialogue choices were pointless because it led to the same conclusion. I even tried to fall all the speech checks, and in the end I still managed to convince her to step inside.

Here's the problem: after she becomes clean, she dislikes you every time you use a drug.

The moment she stepped out of that room I knew something was wrong. She seemed happy. She started smiling like an idiot. I am very disappointed that there are no choices available to the player for the entire course of this quest. There should be a 'dark' option where the player does not care about her long term health and should encourage her to continue using psycho. Hell, I have a large surplus of drugs that I could supply her. That could be a game mechanic: if you cure her addiction, she becomes 'good' and dislikes you for using. If you encourage her to keep the addiction, then she nags you from time to time to give her some drugs, and you have to transfer some to her inventory.

Right now, I am forced to turn around and exit the vault without completing the quest, because I don't want her to change. Bethesda, more choices plz.

Well, it's killing her.

Not much ambiguity there that she's about to die from Psycho overdose.

You shouldn't be popping pills and injecting psycho anyway. Power Armor is greater. And she still likes it when you drink so that's a plus.

Hear, hear! I loved killing Gouhls with Cait, but I love PsychoBuff even more.

Making the player choose between story and gameplay is just bad design.

Well of course she hates it when you do drugs after her personal quest. She sees it as hugely hypocritical that the both of you went through all that trouble to clean her up only for you to pump yourself up with the same stuff that, as she stated herself, was slowly killing her.

So don't tell her you'll help her in the first place!

While this is clearly a valid option, the problem is that this seems to be the solution for a very significant number of quests in the game. When the primary choice that the player is presented with in these quests is participating in linear content or ignoring it entirely, that's a disappointing step backwards compared to previous Fallout titles.

After she's clean she's in recovery. Sort of. The last thing she wants to see are drugs.

Exactly. She's probably thinking: "What the hell? I told you the drugs were killing me, we just went through all that [censored]e JUST to get me cleaned up and now here you are doing the exact thing I clearly told you was bad in that I was gonna die from it? I ought to pound your [censored] in so hard that your grandmother from 210 years ago could feel it!!"

Post-personal quest, Cait's in recovery. Doing drugs in front of her would be like binge drinking alcohol in front of a former alcoholic.

You have a choice to not do the quest if it means that much to you. Do you want "choice" or do you really want XP?

You have the choice: don't do the quest. It would make no sense that you go on a quest to cure her only to say "Let's not cure you!". That would not be choice but BS.

The game has choices where necessary, it just doesn't shove them into your face just to tell you "Hey, we have choice and consequence!".

this,

and this,

Choices can be important, but they also have to make sense, having choices for the sake of choices is bad game design.