I too take Nexium and as I take Magnesium when necessary for cramps, I was interested to know the answer to this myself.

I went into the drug site that I find to be the most comprehensive to suit my own needs, as well as several others I found in my search engine, and I even went to the leaflet in the Nexium box, but no-where could I find any mention of an interaction between nexium and Magnesium.

Here's the link I mostly use to check on drugs, but there are many more available on the Net.

http://www.rxlist.com/cgi/generic/esomeprazole_ad.htm

When you say you take Magnesium Citrate, is this for cramps? I was just wondering because I take Magnesium Aspartate, not a citrate or a chloride.