Homura's not forcing herself not to breathe, but she's also not really bothered by the effects of this crap either. Magical Girls aren't quite human to begin with, so why should drugs work on them?
"A lady huh? Sorry, but you look more like a monster from where I'm standing... So, this is the part where we start fighting each other right? Um, hopefully I can take you in alive, I've never had to fight many people, but um I won't let you hurt anybody!"
She groans at Kotori... Geez seeds? Hobbies are nice and all, but is this really the time for that?
As for what, it seems fairly clear to Homura what's going on here.
"I presume she's here to rob this store Rise-chan, or that's the only reason I can think of for such a thing... Isn't that right, Marry Jane?"
Despite not liking it, she's not willing to strike first against something this akin to a human... Geez, it's so much easier with Witches, former people or not, death is nothing but mercy for such beings. This? She seems sane enough really.