@suzushka Yes, I do that also now! I recently decided that since they get sick at the most inconvenient times and sometimes I'm testing a lot with one of my pets moved into the household or something and am in a hurry or something like that, so I had my sim visit a clinic and purchase a bunch of treats and one ambrosia treat, then uploaded them into a room in my library. That way I can just place the room on the lot, grab the treats I want for the current time or to keep in their inventory, then delete/sell back whatever I don't want. (I uploaded the room to the gallery also, if you want it... user SheriGR).
I've found that if I place the treats into all of the household members' personal inventory, when one of their pet turns up sick I can have that household member (whoever is on the lot at the time) feed the treat to cure the pet. I also can't remember any of those times that it didn't work. Possibly if you waited too long and the pet was super sick/near death? I guess then the ambrosia one may help. I've never needed it yet, but I play with aging off.