Tool · Supplements & Habits
Magnesium Dosage Helper
Which form to buy, how much to start with, and when to take it — based on your goal and what else you're taking. With the "magnesium oxide is poorly absorbed" conversation spelled out.
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What this actually does
Magnesium is essential, but the form you take changes absorption dramatically. Magnesium oxide is about 4% bioavailable; magnesium glycinate, citrate, and threonate are 30–50% bioavailable in published studies (Walker et al., 2003; Lindberg et al., 1990). The same 400 mg tablet can deliver 16 mg or 200 mg of actual magnesium depending on the form.
The recommendation maps your goal to the form with the best evidence for that use: glycinate for sleep and anxiety (gentle on the gut, calming glycine), citrate for general use and constipation, threonate for cognitive concerns (crosses the blood-brain barrier in animal studies; human evidence is thinner), oxide is left for constipation only.
Who it's for
Adults considering magnesium supplementation for sleep, anxiety, or muscle cramps — and who want to avoid the common mistake of buying cheap oxide and concluding "magnesium doesn't work for me" after a week of stomach upset. Also for anyone on common medications who needs to check whether a supplement conflicts.
Limitations
Magnesium supplementation shows modest effects in meta-analyses — most studies find small improvements in subjective sleep quality and anxiety, not dramatic ones. If you have a diagnosed deficiency (which most people don't, but which shows up in people on PPIs, type 2 diabetes, or chronic alcohol use), the effect can be much larger. We don't measure serum magnesium here because it's a poor indicator of total body stores.
This tool doesn't replace a blood test or a conversation with a clinician — particularly if you're on medications, pregnant, or have kidney disease (impaired kidneys can't clear excess magnesium).
Why "oxide" gets a hard time
It's the cheapest form and the one most "complete multivitamin" tablets use. Two real problems: poor absorption (about 4% in fasting conditions; up to 30% with a meal, but still lower than other forms) and a strong osmotic laxative effect. People who take oxide for sleep often conclude magnesium doesn't help them — what they actually took was a mild laxative that didn't reach the brain in meaningful amounts. Glycinate or citrate at 200–300 mg is the more useful comparison.
FAQ
Is magnesium safe to take every night?
For most adults, yes — within the 350 mg/day supplemental upper limit (US Institute of Medicine). Healthy kidneys excrete excess. Stop if you get loose stools; that means the dose is too high for you, not that you need to stop entirely.
Should I get a blood test first?
Serum magnesium is a poor marker of total body magnesium — only 1% of body magnesium is in blood, and the body tightly regulates it. If your level is low on a blood test, you're genuinely deficient; if it's normal, you could still be marginal. A clinician who suspects deficiency usually treats empirically for 1–2 months rather than chasing the test.
What's the deal with magnesium threonate?
Magnesium L-threonate was developed at MIT and shown in animal studies to raise brain magnesium more effectively than other forms. Human evidence is still thin (a couple of small trials in older adults with cognitive concerns). For sleep and anxiety, glycinate has more human data behind it.
Can I take it with my SSRI / PPI / antibiotics?
It depends, which is why the warning box appears if you tick those. Magnesium can bind to antibiotics and reduce absorption if taken within 2 hours of each other; PPIs long-term can deplete magnesium (a real concern worth flagging to your prescriber); SSRIs don't directly conflict but tell your prescriber everything you're taking. The tool reminds you to check with a pharmacist — that's not a hedge, it's the right move.
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