My understanding is that you need about 500 MG per day but you get magnesium from various sources in a healthy diet so you can start with 250 mg and go from there. I've been on 250 per day for a few weeks now and where I used to have literally hundreds of PACs and PVCs each day (previous post) now I'm down to a few dozen maybe and some days none at all. I wish Iknew about it years ago. Overdosing on magnesium can cause diarrhea.