Interesting read...
"CONCLUSIONS: Magnesium hydroxide, administered 1 hour post-iron ingestion at an oral dose of 4.5 g per g elemental iron ingested, significantly reduced iron absorption during a 12-hour period following simulated mild iron overdose in healthy adult human volunteers."
http://www.ncbi.nlm.nih.gov/m/pubmed/9862585/
"CONCLUSIONS: Magnesium hydroxide, administered 1 hour post-iron ingestion at an oral dose of 4.5 g per g elemental iron ingested, significantly reduced iron absorption during a 12-hour period following simulated mild iron overdose in healthy adult human volunteers."
http://www.ncbi.nlm.nih.gov/m/pubmed/9862585/