The short answer
- For sleep: not proven
- Three small trials in older adults found people fell asleep about 17 minutes sooner. The reviewers rated the evidence low to very low quality.
- For mood and anxiety: not proven
- There is no good trial evidence that a supplement lifts mood in women who already get enough magnesium from food.
- For constipation: might help
- Higher doses loosen the bowel. That is a well known effect of magnesium, and it is also its main side effect.
What it is
A mineral you get from food. Spinach, nuts, seeds, beans and wholemeal bread are good sources. Supplements come in several forms, such as oxide, citrate and glycinate.
What the studies say
- A 2021 review looked at oral magnesium for insomnia in older adults. It found three trials in 151 people.
- In those trials, people taking magnesium fell asleep about 17 minutes sooner than people taking a placebo.
- Total sleep time was about 16 minutes longer, which was not a statistically significant result.
- The reviewers judged all three trials to be at moderate to high risk of bias, and rated the evidence low to very low quality. They said the research is not good enough for doctors to make confident recommendations.
- Most people get enough magnesium from food. The UK guideline is 270mg a day for women aged 19 to 64.
How much people take in trials
The insomnia trials in that review gave magnesium for about 8 weeks. The US upper limit for magnesium from supplements is 350mg a day for adults, on top of what you eat.
This is what the research used. It is not a dose for you.
What a label is allowed to say
UK law only lets a supplement say what is on the official list. For this one, that is:
- Magnesium contributes to a reduction of tiredness and fatigue
- Magnesium contributes to normal muscle function
- Magnesium contributes to normal functioning of the nervous system
- Magnesium contributes to normal psychological function
- Magnesium contributes to the maintenance of normal bones
There is no approved claim for sleep. A label that says magnesium helps you sleep is going further than the rules allow.
In the US the rules are looser. A label may say how a nutrient supports the body if the company holds evidence for it. The FDA does not check the claim first, which is why US labels carry the line saying the FDA has not evaluated it.
Safety
- More than about 400mg a day from supplements can cause diarrhoea, nausea and stomach cramps.
- If your kidneys do not work well, magnesium can build up in the blood. That is the one risk worth taking seriously.
- Magnesium can get in the way of some antibiotics, including tetracyclines and quinolones, and of bisphosphonates for bone. A gap between doses usually sorts it, and a pharmacist can tell you how long.
- Some medicines change your magnesium level rather than the other way round. Water tablets and long term stomach acid tablets both can.
Ask your doctor first if
- You have kidney disease or reduced kidney function.
- You take antibiotics, bisphosphonates, water tablets or stomach acid tablets.
- Your nights have been broken for weeks and nothing you have changed has helped.
This is general information, not medical advice. Check with your doctor or pharmacist, especially if you take medication.