While obtaining high levels of magnesium from food is safe, the maximum supplemental dose of magnesium is 350 milligrams per day for adults, and overdoses can be fatal, cautions WebMD. The recommended dietary allowance for magnesium from both food and supplements is 320 milligrams daily for females age 31 and older.
The recommended dietary allowance, or RDA, for magnesium derived from all sources is 420 milligrams daily for males age 31 and older, according to WebMD. Signs of a toxic overdose include low blood pressure, muscle weakness, diarrhea and nausea. The upper daily dose limit of magnesium supplementation for children ages 1 through 3 is 65 milligrams, with an 80 milligram maximum RDA from all sources. Children ages 4 through 8 should take no more than 110 milligrams per day in supplement form, with no more than 130 milligrams per day derived from all sources.
Individuals using heart medications, antibiotics or diuretics and those with diabetes, heart disease or kidney disease should consult with their doctors before taking magnesium to ensure that there are no dangerous interactions or other risks, advises WebMD. Supplements can sometimes cause soft stools as well as cramping and nausea.
High-magnesium food sources include dark green leafy vegetables, beans, peas, nuts and whole-grain cereals, notes WebMD.