I think in the most layman’s terms it is turning off some pain receptors for the dog, which could be controversial I suppose. It can impact kidney function as well. My dog also is experiencing skin irritation (one small hot spot after the first injection, a larger one on the other side after the second injection) - she is prone to allergies and skin irritations so I will be asking my vet at this coming up appointment. I’ve just been using bag balm on the hot spots and that seems to help.
So - like any drug I’ve encountered - it has side effects that may not be desirable. But - to your point it’s Librela or euthanasia for us too so the choice is easy unless the side effects get worse to a detrimental level OR it stops being so effective for her.
I did not notice any real decline in her demeanour or mobility after the first shot (some wonky steps, especially after laying for a long time but nothing long lasting). Like I mentioned, we are now approx 3 weeks post second shot and she’s got some wobbles - more so than after the first one but I also chalk that up at least in part to the changing seasons. My knees and hips HURT the last few days so I bet she’s got some of the same going on. I think a few days either way wouldn’t hurt between shots but I wouldn’t prolong it to something like 6 weeks because I think it’s one of those compounding effect drugs (that’s probably the wrong way to say it) where you have to consistently take it for the drug to work best.
My vet did say that for some dogs it takes 2 shots for the full impact to be noticed but, like you, it was miraculous how much better she got after the first shot.
Edit to add: Since I know many of the dogs who are patients of my vet (pretty much everyone at the barn uses them, plus the vet themselves has 5 large dogs) I can say that quite a few of them are using librela for their older dogs and we have ALL experienced remarkable results.