Magnesium

Adults need 310–420 mg of magnesium a day from food and supplements combined, depending on age and sex. The separate 350 mg/day upper limit applies only to supplements. Check the elemental amount on the Supplement Facts panel, not the compound weight: 500 mg of magnesium glycinate contains only about 55–70 mg of actual magnesium. Most people also get a share from food.

Probably less than marketing suggests. A 2017 review found the evidence that one magnesium salt is reliably better absorbed than another is inconsistent. A 2022 ten-day trial in young women found oxide, often called poorly absorbed, raised ionized blood magnesium, while carbonate lowered it. Several head-to-head comparisons are funded by makers of the product they favor, so read them with that in mind.

The evidence is mixed and mostly weak. One 8-week trial in 46 older adults with insomnia found 500 mg/day improved time to fall asleep and sleep efficiency versus placebo. Industry-funded magnesium L-threonate trials show some benefit, but one 6-week trial found no difference on any objective sleep-tracker measure. A 2026 systematic review rated the overall certainty low to very low.

Yes, at higher doses. Loose stools, cramping and nausea are the most common effects, and they are usually mild and clear up when you lower the dose or stop. A 2023 review found seven of ten studies at 128–1,200 mg/day showed no significant difference in diarrhea versus control. Serious magnesium buildup is rare and occurs almost only when kidney function is already substantially reduced.

Yes. Magnesium can bind tetracycline and fluoroquinolone antibiotics in the gut and reduce their absorption; the usual advice is magnesium 4 hours before or 2 hours after the antibiotic. A small trial found it also lowered levothyroxine absorption, and bisphosphonates are thought to be affected similarly. Loop and thiazide diuretics can increase magnesium loss. Ask a pharmacist about your specific medications.