Nexium is a PPI which works on acid reflux by reducing the acid production in your stomach. PPIs are proven to work very well on acid reflux, but are often underdosed (especially in children), and need to be taken when the proton pumps are active (around meal time). The also can have a limited time in which they are effective. Zantac is an H-2 blocker, and although can work very effectively for some individuals, other individuals build up a tolerance to it very quickly. It seems like the next step is to report to your doctor the fact that you are still having acid breakthough on your current dosage. He or she may want to change your dosage or add a different drug to your regimen. Also, Jeff Phillips has a great chapter in our book Acid Reflux in Infants in Children that explains how each of the drugs work, and also shortcomings of each. Hope this helps!