I think this started, at a low level, Tuesday morning: Southmoon has been behaving very aggressively toward Huckleberry. Hissing, growling, chasing him around the house, that sort of thing. This is entirely out of character; Huckleberry has occasionally been aggressive toward Southmoon in the past, but this week he hasn't been doing anything to provoke her.
Last night, it got intense. This morning, she seemed perfectly normal, right up until Huckleberry jumped down from my desk and landed sort of near her - and then she went into evil mode.
I just checked the package insert for the Revolution, and I see nothing about causing hallucinations in cats. And yet, she's acting like she found the special glasses and can now see that her brother is actually a pod creature from Neptune.
I'd suspect that the flea treatment made him smell different, except that she's also been doing the "who are you and what have you done with my brother" thing from the far end of the hallway.
May have to call the vet this morning.
Update: I'm guessing it's psychological, not pharmacological, and that it's a side effect of Sunday afternoon's flea bombing - or, rather, of the frightful two and a half hours spent in the back yard.
Top Hat (who spent most of that afternoon cowering between shed and fence) remains happy to bound out the door and explore the yard. Huckleberry, normally the bold one, takes a while to venture beyond the patio, and (uncharacteristically) comes scampering back in at the first suggestion that the door might be about to close. Southmoon, who's always been quick to dart through an open door, now seems afraid to get near the door when it's open.
So, maybe she's getting territorial because she was dispossessed (along with the rest of us) for an afternoon?
Update 2: ... Or maybe it was the Revolution? Thursday night, Southmoon slept in the bedrooms-guarding spot on the hallway floor, the better to keep the back of the house safe from the forces of Huckleberry. In the morning, I found on that spot a slightly-chewed cardboard box - I don't know where she found it - which had once contained 6.5x55mm ammunition. Well, at least she didn't have live ammunition, nor a Swedish Mauser to use it in.
Twelve and a half pounds of unpredictable cat with PTSD. Great. But she seemed fine this (Friday) morning (beginning sometime in the middle of the night, when she came bounding into the bedroom), up until she caught sight of Huckleberry, and the growling and aggressive behavior started up again.
Update 3: Later Friday morning. Got home from walkies and errands; Southmoon came bounding up to greet me like a little golden retriever (and not armed for moose). I unpacked a new toy from the shopping bag and commenced waving it around; Southmoon and Huckleberry seemed to be OK in close proximity, even when he waved a paw in her face while swatting at the toy. But then she started sniffing his neck, and the growling started up again. Hm?
O-kay, then. Moisten a paper towel, and give Huckleberry's neck a good scrub (after consulting the Revolution instructions and confirming that it's fine to give the critter a bath after a couple of hours). Southmoon seems calmer now. Interesting.