Not sure Danielle.. the pediatrician just mentioned it becomes inactive or something once its cut and left, so to make sure i discard whatever isnt used (DD2 is on half a tablet once a day). So sometime within that 24 hours it becomes inactive.. im guessing not long after once the granules are exposed to air.. so if you find your DS is a bit more irritable after the 2nd half of the tablet your giving in the morning, it might be why?