We started my daughter on it when she was almost 6 months and the result was practically INSTANEOUS. No more blood in stool, no more painful BMs, no more painful gas, no more stomach aches, etc., within a weekend's time. So quick, that her GI was surprised, telling us that he usually sees full effect in about 1-2 weeks like others have said. My only complaint is the cost, as I'm sure the other moms will join me in this lament. She has fattened up really well since then, but I don't know if that's also because she started on solids, too, but her doctors are all very happy with her weight gain/growth. I don't think there was an explosive growth, as even when she had all the symptoms her weight gain was steady, but there was definitely an increase. After a week of regular poop (yes, watch out for the funky dark green smelly stuff), her constipation started and lasted for about 2 weeks, but she is now pretty regular. We do an oz. or 2 of diluted prune juice (1 to 1 ratio) every other day (in the beginning it was every day) or so, when we think she needs it. Good luck!