Magnesium glycinate side effects and safe amounts

What are the side effects of magnesium supplements?

The main one is loose stools or diarrhoea. It's the usual side effect of any magnesium supplement and becomes more likely as the dose goes up. UK product information for a licensed magnesium medicine lists diarrhoea, says it usually depends on the dose, and advises cutting the dose if it happens. Serious problems are rare and mostly affect people whose kidneys don't work well; they are covered further down.

What are the side effects of magnesium glycinate?

Glycinate (also sold as bisglycinate) caused few problems in its largest recent trial. In that 2025 trial, published in Nature and Science of Sleep, 77 adults took 250 mg of magnesium a day as bisglycinate for four weeks and 76 took a placebo. Nobody in the magnesium group reported gut problems. That group reported two side effects in total, one case of dizziness with nausea and one of joint pain, while the placebo group reported seven, three of them gut-related. It was one short trial in healthy adults, and people with kidney or bowel disease weren't allowed to take part. For how glycinate compares with citrate and oxide in the gut, see our guide to magnesium glycinate vs citrate.

Can magnesium cause diarrhoea?

Yes. Magnesium that your gut doesn't absorb stays in the bowel and holds water there, which loosens stools. Magnesium citrate does the same thing on purpose as the osmotic laxative in some UK bowel-preparation medicines. The European Food Safety Authority (EFSA) based its upper level for supplemental magnesium on studies in which people developed mild diarrhoea, and the NHS says taking more than 400 mg for a short time can cause it.

If magnesium upsets your stomach:

• Lower the dose, or split it between two meals.
• Take it with food. An NHS hospital guideline on treating low magnesium advises giving oral magnesium with food where possible, to reduce diarrhoea.
• Check for doubling up. A multivitamin, an electrolyte drink, or a laxative or antacid containing magnesium all add to your total.
• If it carries on, stop and ask a pharmacist. The NHS says to call 111 if diarrhoea lasts more than 7 days.

Can magnesium help with constipation?

Magnesium can loosen stools, which is why some magnesium salts work as laxatives, but a daily supplement is not a treatment for constipation. The difference is the dose. In two small four-week trials in Japan, adults with chronic constipation took 1.5 g of magnesium oxide a day. In the larger one, which also tested senna, overall symptoms improved in 68% of the magnesium oxide group and 12% of the placebo group. At the oxide figure used below, 1.5 g is about 900 mg of magnesium, more than twice the NHS guide for supplements, so it is a medicine-sized dose rather than a supplement. Magnesium citrate is the osmotic laxative in some UK bowel-cleansing medicines.

The NHS says to speak to a pharmacist if diet and lifestyle changes aren't helping, that most laxatives work within 3 days, and that they should only be used for a short time. See a GP if constipation isn't getting better with treatment, or if you have blood in your poo, tummy pain or unexplained weight loss.

Is 500 mg of magnesium too much?

If it's 500 mg of magnesium itself from supplements, it's more than UK guidance suggests. But a label that says 500 mg may not mean 500 mg of magnesium.

The NHS says 400 mg or less a day of magnesium from supplements is unlikely to cause any harm. EFSA's upper level is lower, at 250 mg a day: the amount that didn't cause diarrhoea in the studies it relied on. Neither figure includes magnesium that's naturally in your food. For context, the NHS says adults need 300 mg of magnesium a day (men) or 270 mg (women) in total, and that a varied, balanced diet should provide it. Our list of foods high in magnesium shows where it comes from.

Some packs give the weight of the whole compound rather than the magnesium in it. Glycinate is only about 14% magnesium by weight: in the 2025 trial, each 893 mg capsule of magnesium bisglycinate supplied 125 mg of magnesium. Oxide is about 60% magnesium: in a 2012 trial, each 414 mg tablet of magnesium oxide supplied 250 mg. UK rules require supplement labels to state the amount of each mineral in the product, so look for the magnesium figure in the nutrition table.

Is 500 mg of magnesium glycinate too much?

Usually not, if 500 mg is the weight of the compound. That gives roughly 70 mg of magnesium, well inside both limits. If the label means 500 mg of magnesium itself, it's above the NHS guide and EFSA's upper level, and loose stools become more likely. Our glycinate vs citrate guide explains how to read a UK label in more detail.

Can you take too much magnesium?

From supplements, yes. From food, it's very unlikely if your kidneys work normally. Healthy kidneys get rid of magnesium you don't need, and EFSA's upper level applies only to magnesium from supplements, water and fortified foods, not to magnesium that's naturally in food. The NHS adds that there isn't enough evidence to say what high doses do over a long time.

Dangerously high blood magnesium, called hypermagnesaemia, is uncommon. A review in Clinical Kidney Journal notes that low magnesium is seen more often than high. High levels can develop in advanced chronic kidney disease, or when people take too much magnesium as supplements, laxatives or antacids, particularly older people whose kidney function is declining. An NHS hospital guideline says it's more common when magnesium is given by drip or to people with kidney impairment, and UK product information for a licensed magnesium medicine links it to higher doses and impaired kidney function.

Signs of too much magnesium

UK product information for magnesium lists flushing, thirst, nausea and vomiting, drowsiness, confusion and low blood pressure. At higher levels come muscle weakness, loss of reflexes, breathing problems and an irregular heartbeat, and in severe cases coma or cardiac arrest.

Who should not take magnesium supplements?

Most healthy adults can take a supplement within the NHS guide. Check with a GP or pharmacist first if any of these apply to you:

• You have kidney disease. Poor kidney function is the main reason magnesium builds up, and UK product information for a licensed magnesium medicine rules it out in severe kidney impairment. Don't take a supplement unless your kidney team advises it.
• You have myasthenia gravis. An NHS hospital guideline notes that magnesium may make its symptoms worse.
• You take tetracycline antibiotics such as doxycycline, a bisphosphonate such as alendronic acid, iron tablets or penicillamine. Magnesium can reduce how much of these you absorb, so they need spacing a few hours apart. The FAQ below gives the gaps from UK product information, and your pharmacist can give the right one for your medicine.
• You've taken a proton pump inhibitor such as omeprazole for more than a year, or you take diuretics. These can lower magnesium, and the MHRA has advised doctors to consider checking levels. Ask for a test rather than treating yourself.
• You're pregnant or breastfeeding.

How long does magnesium take to work?

There's no set timescale. The magnesium itself reaches your blood within hours: a 2017 review says absorption starts about an hour after a dose and is about 80% complete at six hours. How long it takes for magnesium glycinate, or any other form, to work for you is another matter, and it depends on what you're hoping it will do. An adult body holds about 24 g of magnesium, almost all of it in bone, muscle and other tissues rather than in the blood. A supplement adds to that store gradually, and a blood test gives only a partial picture of it.

The 2025 bisglycinate trial lasted four weeks. Most of the fall in insomnia scores came in the first two weeks, but it happened in the placebo group too, and magnesium came out only slightly ahead at week four. So if you've taken a sensible dose daily for about a month and noticed nothing, it probably isn't doing much for you. Our guide to magnesium and sleep looks at that evidence in full. If a doctor is treating low magnesium, they'll recheck your blood rather than go on how you feel.

What is the best time to take magnesium?

The best time to take magnesium is whenever you'll do it consistently. No trial we found shows that morning beats evening. If you're wondering what time of day to take magnesium, these points matter more:

• With a meal if your stomach is sensitive. An NHS hospital guideline for treating low magnesium advises giving oral magnesium with food where possible to reduce diarrhoea. In one study, about 52% of the magnesium in mineral water was absorbed with a meal against about 46% without.
• In smaller doses if you take a lot. A 2017 review found that a larger share is absorbed when the same amount comes in several small doses, so split bigger amounts across two meals.
• In the evening if it's for sleep, which is how the 2025 trial did it (30 to 60 minutes before bed). Our guide to magnesium and sleep covers that timing.
• A few hours away from some medicines. UK product information for a licensed magnesium medicine advises 2 to 3 hours from tetracycline antibiotics such as doxycycline, and 3 to 4 hours from bisphosphonates such as alendronic acid, iron and penicillamine. Ask a pharmacist for your gap.

How should you take magnesium?

Follow the serving size on the label, and work from the magnesium figure in the nutrition table rather than the weight of the compound. If you get loose stools, lower the dose, as UK product information for a licensed magnesium medicine advises, or split it between meals. A powder or liquid is measured and mixed as the label says; our guide to magnesium glycinate vs citrate covers how powders and liquids compare with capsules. Add up the magnesium from every product you take, and check with a GP or pharmacist first if you have kidney disease or take any of the medicines listed above.

Can you take magnesium after drinking alcohol?

A normal dose after a drink isn't known to be harmful, but magnesium won't prevent or cure a hangover. A 2022 systematic review in the journal Addiction looked at 21 placebo-controlled trials of hangover remedies and found only very low-quality evidence for any of them. Magnesium wasn't among the remedies that showed an effect.

Long-term heavy drinking makes you lose more magnesium in your urine, and an NHS hospital guideline lists chronic alcohol misuse among the causes of low magnesium. If that sounds like you, start with your GP, who can check your blood and help with the drinking. And if a night out has left your stomach unsettled, a magnesium supplement could loosen things further, so it can wait until you've recovered.

Does magnesium help with weight loss?

There's no good evidence that it does, and no magnesium product in Great Britain may claim it. None of the authorised health claims for magnesium mentions weight.

Two systematic reviews published in 2021 pooled the trials. One, in the British Journal of Nutrition, combined 28 randomised trials with 2,013 participants and found no significant change in body weight, BMI, waist size or body fat overall. Waist size was slightly lower only in a subgroup of people with obesity, and the authors called for better studies. The other, in Critical Reviews in Food Science and Nutrition, combined 32 trials and also found no significant change in body weight. It did report a very small drop in BMI, which only just reached statistical significance and came mainly from people who were low in magnesium, had obesity or had insulin resistance. Results varied a lot between the trials, which used 48 to 450 mg of magnesium a day for 6 to 24 weeks.

If weight is what you're working on, our guide to appetite suppressants in the UK covers what has evidence, including the prescription options to discuss with a GP.

Where Natura fits

Natura's Daily Magnesium (coming soon) is one capsule giving 200 mg of magnesium as glycinate, within both the NHS guide and EFSA's upper level. Magnesium contributes to normal muscle function and to the reduction of tiredness and fatigue. You'll find it among the single bottles. Daily Electrolytes and several Natura packs also contain magnesium, so add the amounts together if you use more than one.