switching formula

tristan only likes to eat similac organic , yesterday he ran out of it and we fed him similac sensitive and then this morning he had the advanced formula there the same brand name but different and i know they say not to switch formula does that mean like brand name and all

Yes, that means you shouldn't switch formulas at all. The sudden change can upset their stomachs and cause GI distress.

My oldest had an allergic reaction to a formula when we bought the wrong can. Same brand, just one with lipil and one without and it was enough to cause a problem.