Changing cat foods..?

I feed Natures Variety Instinct Chicken flavor and one day accidentally bought Natures Variety Prairie Chicken flavor. They are the same food but Prairie have grain. So, I told myself I would change back after we finished the Prairie. I am shifting back but want to finish my Prairie. Here is my issue: My sensitive cat who has vomited all his time has been vomiting more lately. I am guessing I should just switch over completely since I would fairly not save money instead of the poor guy vomiting and the cleanup needed. Luckily just now he be about to vomit on my computer desk. Luckily we got him onto the floor in time. I agree to him out for a while when I get home and then nurture him. you think maybe he is eating grass and that I should nurture him first before going outside?
i have a cat who had the same problem next to vomiting, but if u put hair ball tablets on his paw it helps..(or butter) make sure he dosent fling it bad his paw though.lol it help with vomiting some how... and cats are brats and make certain u know when they dont like something. so if he wasnt vomiting with the prehistoric food then u should stick with it. and as far as eating grass it help with hairballs so its good for cats.
hope i helped=)
You should stop letting him go outside if you want the vomiting to stop, my cat LOVES lettuce and grass and adjectives those greens. But whenever we feed him some, he barfs it up. I don't know what it is, maybe some kind of a food allergy.

Stop letting him run outside, give him no access to greens. Just feed him regular kitty food for a while, and see what happen.

Again, it's probably the grass.
Answers:    My cat has not long started vomiting food but otherwise appears fine. The vet told me they eat grass to make themselves sick so it is not the grass that is the root of the problem. I changed her diet to Hills Sensitive Stomach and the vet have given her Pro Kolin (think in the usa it is called protexin) which is a paste I spread on her paw (so she licks it). It acts like a probotic yogurt would for us ensure she has healthy bacteria contained by her gut. I was at my wits end near her barfing and my carpet was starting curl but it seems to hold stopped. Worth a try.Good luck.
Usually animals eat grass to unemotional their stomach. So I would feed beforehand. With our cat we just leave food out adjectives the time, she doesn't over eat.
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