The short answer
Most adults who need a supplement take 100 to 300 mg of elemental magnesium a day, with food, in one or two doses. Stay at or below 400 mg a day from supplements unless a doctor advises more. Keep it two to four hours apart from medicines it can block, and give it about a month before judging whether it helps.
On this page
Do you need a supplement at all?
Start there. Most adults can meet their needs from food, and the Department of Health and Social Care says a varied diet should provide enough. A supplement makes most sense if your diet is limited, you are in a higher-risk group, or you are trying magnesium for a specific reason with some evidence, such as migraine or sleep.
How much should you take?
See dosage and timing for amounts by purpose. The short version:
| Purpose | Typical daily elemental magnesium |
|---|---|
| Topping up a low intake | 100 to 200 mg |
| Sleep (as in trials) | 250 to 500 mg |
| Blood pressure (as in trials) | 300 to 400 mg |
| Migraine prevention (as in trials) | about 600 mg, with medical advice |
| Constipation | as directed on a licensed laxative |
The NHS says 400 mg a day or less from supplements is unlikely to cause harm.
With food or without?
With food is better for most people. It reduces stomach upset and diarrhoea and, for forms like oxide that need stomach acid to dissolve, may help absorption. If you take more than about 200 mg, splitting it into two doses is gentler.
What should you keep it apart from?
Magnesium can bind to some medicines in the gut and stop them being absorbed. Leave a gap of two to four hours between magnesium and:
- tetracycline antibiotics (such as doxycycline) and quinolones (such as ciprofloxacin)
- bisphosphonates for osteoporosis (such as alendronic acid)
- levothyroxine
The patient leaflet for each medicine says how long a gap to leave. See medicines it affects for the full list.
What goes well with it?
- Vitamin D. The body needs magnesium to activate vitamin D, and the NHS advises everyone in the UK to consider 10 micrograms a day in autumn and winter.
- A balanced diet. The foods richest in magnesium also bring fibre and potassium.
How long until it works?
As a laxative, hours to a day. For sleep and blood pressure, trials ran for three weeks to three months. For migraine, give it about three months. If you notice no difference after a fair trial, stop.
In this section
- Magnesium dosage: how much to take and when
How much magnesium to take for sleep, cramps, migraine, blood pressure and constipation, the safe upper limit and the best time of day.
Common questions
Should I take magnesium in the morning or at night?
It makes little difference for most purposes. If you are taking it for sleep, trials gave it in the evening. If it upsets your stomach, take it with a meal.
Can I take magnesium with vitamin D?
Yes. There is no harmful interaction, and magnesium is needed for the body to process vitamin D. In the UK, the NHS advises everyone to consider 10 micrograms of vitamin D a day in autumn and winter.
Can I take magnesium with calcium or zinc?
Yes, but very high doses of calcium or zinc taken at the same moment may slightly reduce magnesium absorption. If you take large doses of each, spreading them through the day is a simple precaution.
How long can I take magnesium for?
At up to 400 mg a day, healthy adults can take it long term. Review it every few months: if you notice no benefit, you may not need it.
References
- NHS. Vitamins and minerals: others (magnesium).nhs.uk
- NHS. Vitamin D.nhs.uk
- National Institutes of Health Office of Dietary Supplements. Magnesium: fact sheet for health professionals.ods.od.nih.gov
- Rosanoff A et al. Essential nutrient interactions: does low or suboptimal magnesium status interact with vitamin D and/or calcium status? Advances in Nutrition, 2016.pubmed.ncbi.nlm.nih.gov
First published 4 November 2026. Spotted an error? Tell us and we'll correct it. Read our editorial policy.