Alimentum

My 5 week old baby boy was fussy and having very runny poopies a week ago, and the doctor gave us ready to feed samples of Alimentum formula (Similac) to see if he has a food allergy. Well, immediately, his poops got better (more peanut butter consistency) and he was less fussy. We used all the ready to feed, and things were great... until we began to use powder. When we used the powder Alimentum, now his poops are mucous consistency.

Has anyone else had this experience? Anyone have any advice?