I heard this from a nurse when being told to do a Colon Cleanse for a procedure. I was using mag citrate in a fairly high dose (from those colored bottles of liquid meant to liquefy everything). She said with laxative use one has to be careful not to through off the electrolytes.
I'm not sure that you are using too much, if you were you would be having diarrhea every time you used it. This is where my question about oxy cleanses comes in - I just read recently on curezone that the oxy products actually become mag citrate as they brake down in the body. This concerns me because I know Oxypowder s create "butt Pee" as people have called it, and so does overdoses of Mag citrate. Overdosing on magnesium every day would through off the electrolyte balance. However, it is possible that these products really do work through oxygen, requiring less mag and therefore not throwing off that balance too much.
A side note about Oxypowder , people who have used this product have reported many side effects not found in other oxy products, and the maker seems to recommend a 7 day cleanse with it - the same maximum recommended for the high dose Magnesium laxative products found in grocery stores. This makes me a little suspicious of that particular product, as that the other oxy products say they are safe for long term use.