Magnesium, B6 and vitamin D: what the evidence supports
These are the 3 supplements I get asked about most. Magnesium has decent evidence for a few specific problems and weak evidence for sleep. B6 is the one that worries me, because people take it from 4 bottles without knowing, and too much can damage nerves. Vitamin D matters if you are low or in a group shown to benefit. For most healthy adults under 75, doses above the daily requirement have not shown a payoff. Here is each one, with what I would do first.
Why these 3 end up in every cabinet
Each one has a real job in the body, and each one gets recommended for almost everything. Tired, anxious, crampy, flat, not sleeping. Someone online has a protocol for it with all 3.
I see the result when a client lays the bottles on the table. A multivitamin, a B complex, a magnesium blend for sleep, a hair formula, a vitamin D with K2. Nobody added up what the bottles share. The total dose is the number that matters, and almost nobody knows theirs.
Magnesium: useful for some things, oversold for others
Adults need about 310 to 320 mg a day for women and 400 to 420 mg for men. Close to half of Americans get less than they need from food. So low intake is common, and it is fixable with pumpkin seeds, almonds, beans, spinach and whole grains.
A blood test will not settle it. Less than one percent of your magnesium is in the blood, and the body keeps that level tightly controlled. A normal serum magnesium can sit on top of a low intake.
The evidence is best for constipation. Magnesium oxide has held up in trials, and gastroenterology guidelines include it as an option for chronic constipation. It is also rated as probably effective for preventing migraine. The doses used for migraine are higher than the supplement limit, so that one belongs with your doctor.
For sleep, the trials are small and the quality is low. Some people tell me it helps them. I believe them, and I also tell them the research is thin. For muscle cramps in older adults, the best review found it is unlikely to help much.
The upper limit for magnesium from supplements is 350 mg a day. Food does not count toward that. The first sign of too much is loose stools.
The cautions are specific. If you have kidney disease, do not take magnesium without your doctor, because your kidneys are what clear it. If you take thyroid medication, keep magnesium 4 hours away from it, since it can block absorption. The same spacing issue applies to some antibiotics and to osteoporosis medications. Ask your pharmacist.
B6: the one I worry about
You need very little. Adults need between 1.3 and 1.7 mg a day, and food covers it easily. Chickpeas, fish, chicken, potatoes and bananas all carry it. True deficiency is uncommon in people who eat regular meals.
B6 does have real uses. It is a standard first treatment for nausea in pregnancy, at a dose chosen by the doctor or midwife. If you are pregnant, please use their dose and no one else's. For PMS, older trials suggest it may help some women, and those trials were small.
The problem is the upper end. Too much B6 over time can damage the nerves in the hands and feet. It shows up as numbness, tingling, burning or trouble with balance. It often improves after stopping, and recovery can be slow.
The safe ceiling is under debate. The United States set its limit at 100 mg a day back in 1998. European regulators lowered theirs to 12 mg a day in 2023. Australia reviewed its own cases, found nerve damage reported at 50 mg a day and sometimes lower, and is moving products with 50 mg or more behind the pharmacy counter in 2027.
Stacking is how people get there. B6 is in multivitamins, B complex, magnesium blends, hair and nail formulas, energy drinks and some protein powders. On a label it may say pyridoxine or P5P. Add up every source. I regularly see totals over 100 mg in people who believed they were taking a little.
Vitamin D: who it helps
Vitamin D is essential for bone, and real deficiency needs treatment. The question is whether extra helps people who are already fine.
The Endocrine Society guideline from 2024 answered it more conservatively than most people expect. For healthy adults from 19 to 74, it advises against routine testing and against taking more than the daily requirement. Large trials did not show that extra vitamin D prevented heart disease, cancer or fractures in generally healthy adults.
The guideline does suggest supplementing in 4 groups. Children and teenagers. Adults 75 and older. Pregnant women. Adults with prediabetes at high risk of progressing. If you are in one of those, the conversation with your doctor is worth having.
The daily requirement is 600 IU up to age 70 and 800 IU after that. The upper limit is 4,000 IU a day unless a doctor is managing a deficiency. Very high doses over months can push blood calcium up and lead to kidney stones.
Testing makes sense when there is a medical reason. Osteoporosis or a fracture from a minor fall. A condition that limits absorption, such as celiac disease, Crohn's or weight loss surgery. Kidney or liver disease. Certain long term medications.
A supplement is a dose. Count it like one.
This applies to men and younger readers
Men often meet these 3 in a sports blend sold for sleep and recovery, which combines zinc, magnesium and B6. Add a multivitamin and a pre workout drink and the B6 total climbs fast.
Younger readers get B6 from energy drinks, sometimes several a day. If you are in your twenties with tingling fingers and a daily energy drink habit, read the can.
What I would do first
I would put every bottle on the table and write down what is in each one, with the dose. Include powders, gummies and drinks. This takes 20 minutes and it is the most useful thing in this note.
Then I would add up the B6. If the total is well above what a basic multivitamin provides and no doctor asked you to take it, I would drop the extras. Bring the list to your doctor or pharmacist if you are unsure which to keep.
For magnesium, I would start with food for 2 weeks. If you still want to try a supplement, pick one form for one reason. Oxide or citrate if constipation is the goal. Glycinate is easier on the gut if it is not. Stay at or under 350 mg, give it 4 weeks, and stop if nothing changed.
For vitamin D, I would stay near the daily requirement unless a doctor tested you for a reason and gave you a dose. If you are in one of the 4 groups, ask.
I would show the full list to whoever prescribes for you. This matters most if you take blood thinners, thyroid medication, diabetes medication or water pills, because supplements interact with all of them.
When to push harder with your doctor
Numbness, tingling or burning in your feet or hands, or new clumsiness, needs attention. Stop the B6, see your doctor and ask whether a B6 blood level is warranted. Bring the bottles.
Call promptly if you have kidney disease, take magnesium, and notice muscle weakness, a slow or irregular heartbeat or unusual drowsiness.
Nausea, strong thirst, frequent urination, constipation or confusion while taking high dose vitamin D calls for a calcium check.
Bone pain, a fracture from a small fall or loss of height is a reason to ask for a vitamin D level and a bone density scan.
If you are exhausted and supplements have not touched it, ask for the workup instead of another bottle. Ferritin, a full thyroid panel and fasting insulin are where I would look.
FAQ
Which form of magnesium is best?
It depends on the reason. Magnesium oxide and citrate pull water into the bowel, so they suit constipation. Glycinate tends to be easier on the gut. No form has strong proof of being better for sleep.
Does magnesium help you sleep?
The trials are small and low in quality, so the answer is maybe. It is reasonable to try for 4 weeks at or under 350 mg if your kidneys are healthy. If nothing changes, stop taking it.
How much B6 is too much?
The United States limit is 100 mg a day. Europe set 12 mg a day in 2023, and nerve damage has been reported at 50 mg and sometimes lower. Add up every product you take and keep the total low unless a doctor prescribed more.
Should I get my vitamin D tested?
Current guidance advises against routine testing in healthy adults. Testing makes sense with osteoporosis, a condition that limits absorption, kidney or liver disease, or certain medications. Ask your doctor whether one of those applies to you.
Can I take magnesium with thyroid medication?
Yes, with spacing. Magnesium can block absorption of levothyroxine, so keep them 4 hours apart. Tell your prescriber, because your thyroid levels may need a recheck.
In good health,
Yvette Rose
Sources
- NIH Office of Dietary Supplements. Magnesium fact sheet for health professionals.
- EFSA Panel on Nutrition. Scientific opinion on the tolerable upper intake level for vitamin B6. EFSA Journal, 2023.
- newsGP (RACGP). A long time coming: TGA clamps down on vitamin B6.
- Demay MB et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab, 2024.
- NIH Office of Dietary Supplements. Vitamin D fact sheet for health professionals.
This is general wellness education and not medical advice, diagnosis or treatment. Talk to a qualified professional, especially if you are pregnant, nursing, taking medication or managing a condition.