You can have too much. This is why supplements sometimes suggest starting at 1/2 the RDA. The assumption is that you
are eating some, but not enough. If you were really really low your heart would go into arrhythmia (some people die at this point). People on strong diuretics sometimes end up in the ER with this emergency.

So with SlowMag, that would be 2 tablets a day would be about 1/2 the RDA.

Then there is the elemental issue...as some supplements are not clearly labeled. 400mg of elemental is different than 400mg of chelate total. So when people look at dosing, they may think they are getting more than they actually do.

This is an example of a properly labeled mag product:
http://www.swansonvitamins.com/docto...esium-240-tabs

Things are changing with labeling with time, so more companies put the elemental on them now.

RDA for adults is 350-400mg elemental for magnesium.

For people with normal functioning kidneys, excess is eliminated. Massive doses orally really cause diarrhea, and not much poisoning happens this way. But rectally, in enema form, it can happen and it does happen with babies and elderly.
Fleets PhosPhoSoda was taken off the market for X-ray preps because of toxicity to the elderly or kidney impaired. But it was used for decades!