I know a lot of people interpret Scully's reaction to Mulder's love confession in 'Triangle' as her assuming that he is drugged up and talking nonsense.
There is another (worse) option, though.
"Oh brother" is definitely not what you respond if you're taking it seriously, we can probably agree on that, but i don't think potential drugs are the reasons here.
Let's look back a little. I'm working with the assumption that 'Folie a Deux' is set in May of 1998 with FtF taking place in August. 'The Beginning' happens sometime in September of the same year. If we use this as our timeline, we have a total of around five months during which a LOT of shit happens.
'Folie a Deux' is important because this is the "original" drugged up love confession Scully hears when Mulder is in the psychward. It gets to her, she thinks he is being genuine and does what he asked her to do.
More important than the act itself is what he tells her. I assume most of us have that conversation memorized, but just as a reminder:
"Scully, you have to believe me. Nobody else on this whole damn planet does or ever will. You're my one in five billion."
Even from our point of view, this makes complete sense—she IS the only one he trusts.
Except that it's a lie. I don't expect Mulder to think of Diana in that moment, and it wouldn't have been an issue at all if it weren't for the events of 'The End', which happen about 2-3 weeks later.
Suddenly, there is a person from his past that he trusts unconditionally to the point of stupidity, and he not only doesn't talk to Scully about any of it, he stops talking to her period. It's as if she has been a placeholder for Diana, and now that she's back, there is no use for her anymore.
Mulder works the case with Diana, Scully is dragging herself along and he does not care about her or her work until it is useful for him; he doesn't back her up when a room full of people (including Diana) belittle and mock her. He nods along, which, to Scully, is just the last puzzle piece slotting into place.
To summarize, Mulder's confession in Folie a Deux that she took seriously is revealed to be a lie—a means to an end to get her to do what he wants her to. Whether or not that's objectively true is a totally different matter because this is what Scully is confronted with.
With that in mind, their argument in FtF in his hallway and Scully thinking of herself as redundant makes complete sense. We get yet another love confession, one she ALSO takes seriously, and if it weren't for that fucking bee, they'd have kissed. Maybe then, the whole mess that comes after could have been avoided, but here we are.
In "The Beginning", Scully repeats said confession back to him in an attempt to calm him and regain his trust in her/her science. Let's see what happens:
He implicitly tells her that none of what he confessed still applies, that the whole speech was, once again, nothing but a farce to get her to stay. Mulder confirms this at the end of the episode, and Scully asking him with tears in her eyes to trust her is a deeply painful contrast to his earlier declarations.
Two times, Mulder confesses his love to her.
Two times, he takes it back later and continues to use it against her.
In 'Triangle', Scully once again finds herself next to Mulder in a hospital bed and listens to him as he confesses his love for a third time. Does she think he's drugged up? Yeah, probably, but that's not the reason she cannot take him seriously.
Considering her track record with Mulder and big speeches, is it at all surprising that she no longer trusts what he tells her?
"Oh, brother," because she is forced to grapple with her many, many feelings for him and process it in a way that fits into everything else that has happened so far.
"Oh, brother," because it might be an attempt to placate her and nothing else.
"Oh, brother," because there is a chance he will twist his 'I love you' into a weapon to stab her with later. Because he says it and does not mean it.
This is not a "oh, he's drugged up" expression to me. This is Scully preparing to get hurt again.
The funniest part is that she's right. She is right. He couldn't have meant it because then he wouldn't be staring her down with apathy in his eyes and tell her not to make it "personal".
SHE is making it personal? After Mulder manipulated her by pulling on their connection like a leash to get her to do what he wants? After he confessed how much he loves and trusts her over and over just to throw it all away?
To ignore all of it in the moment it actually fucking matters?
The drug option is less angsty and kinda fun, and I do enjoy reading fics that interpret it as such, but with their history attached, it is so much more—and so much worse.
Pretending it never happened is one thing, but then actively denying it to her face and using her feelings to hurt her? Yeah, no.
Mulder fucked up and Scully has every right to reject his declarations and distance herself from him.