I switched cat food brands about 3 weeks ago, from Royal Canin to Purina Pro Plan, and ever since my cat's been throwing up almost every day. I don't know if it's food he can't tolerate or if there's something more serious going on. He ate the old kibble for years without any problems.
Take him to the vet as soon as possible, because daily vomiting isn't normal and it could be the food or something more serious that needs to be ruled out. Yes, diet changes can cause digestive upset, but usually it passes in a few days if your cat is adapting. If he's still vomiting after 3 weeks, it's more likely that he just can't tolerate that brand or there's something else going on.
If the vet rules out medical issues, then you can try switching back to the old kibble to confirm, or do a more gradual transition by mixing the two foods together. Don't just throw out the new food and grab a different one right away, because then you won't even know if your cat would accept it. If your vet says it's just adjustment, wait a bit longer, but if he's vomiting that frequently it really is weird.
The food change could really be the culprit, especially if the vomiting started right after the switch! But three weeks is quite a while - usually intolerances show up in the first few days, while his body is still adjusting to something new. The fact that he's vomiting almost every day though isn't normal, and honestly I'd try going back first and see if the problem goes away. If the vomiting stops within a few days, then it was definitely the Purina that wasn't working for your cat.
If it continues even after you've switched back to Royal Canin, that means there's something more serious going on - it could be stomach inflammation, a blockage, an allergy that developed in the meantime, or other conditions that have nothing to do with food. In that case you need to take him to the vet as soon as possible, don't wait any longer! Daily vomiting isn't something to brush off.
In the meantime, if you decide to test the old food, do it gradually - mix some of the new with the old for a few days, so his stomach doesn't get shocked. And pay attention to whether the vomiting always happens a while after eating or if it's random. Anyway, if the problem persists after going back to his previous food, the vet is your only option!
Three weeks is weird timing for a simple food intolerance, because yeah, intolerances usually show up in the first few days. If the vomiting's been going on this long, it could be that your cat developed something during this switch (a more serious reaction) or that the vomiting has nothing to do with the kibble at all and it's just a coincidence. I had a similar case with a dog that had been eating the same brand forever, then we switched it and for two weeks nothing strange happened, then suddenly he started vomiting. It seemed like the food, but at the vet visit it turned out he had a foreign object in his stomach - a piece of a toy he'd swallowed who knows when.
The important question is: did the vomiting start the day after the switch or was it gradual? And how's your cat doing generally, aside from this - is he active, eating anyway, drinking normally? If he's vomiting every day and it's been going on like this for weeks, the other people's advice to see a vet is still number one, but I'm not sure just switching the food back will fix it. You could still try gradually reintroducing the old kibble (mixing it with the new stuff for a week or two) to rule out that cause, but if it keeps happening, don't put off the vet visit.
Log into answer.
