My baby has been very fussy since 2 weeks old . So we've been changing formulas and are now on Nutramigen. She was constipated and gassy for awhile so we changed to that to help. Now she has regular bowel movements , but is more fussy and spits up more and screams while we try to feed her and you can tell she is hungry . We went to pediatrician and they prescribed her some Zantac, and told us to give her .5 mL twice a day . I started her on it last night and haven't seen any improvement yet . I also read that Zantac is weight sensitive so you have to keep upping the dose for to stay effective .