Hi Joe - As George mentions, the citrate form when you use it frequently can have a laxation effect. The amino acid chelated form by the Albion patented process is the one serves most of us the best as it doesn't cause bowel tolerance issues until dosing becomes quite high... like around 800 - 1,000 mg. (depends on the individual) and it doesn't break down in the stomach and combine with other elements. The goal is to push to your max dosing as indicated by bowel tolerance since we continually have to keep optimizing our magnesium stores inside the cells. Magnesium is easily and quickly depleted so it's a continual task to remain optimally repleted.
Stay away from any that list aspartate as part of the complex because that tends to be aspartate is an excitotoxin. .
Please read through the Magnesium Absorption report for more details.
Magnesium Absorption
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www.afibbers.org]
At the iHerb website, you'll find Bluebonnet Nutrition, Magnesium Chelate which is by the Albion process and is also made in the USA and has the advantage of being capsules rather than the large tablets. However, many find Doctor's Best works well for them, also, and it's based on on the Albion process plus other ingredients, as I recall.
Hope this helps.
Jackie