Last updated: October 2026
Magnesium supplements are safe for most adults at sensible doses. The most common side effects are diarrhoea, stomach cramps and nausea, usually from high doses or forms like oxide and citrate. The NHS says 400mg a day or less from supplements is unlikely to cause harm. People with kidney disease, or on certain medicines, should check with a doctor first.
Below: the common and rare side effects, whether glycinate can cause diarrhoea, magnesium spray reactions, allergic reactions, high blood magnesium, the normal blood test range, medicine interactions and how to reduce side effects.
Common side effects of magnesium
| Side effect | How common | Why |
|---|---|---|
| Diarrhoea or loose stools | The most common | Unabsorbed magnesium draws water into the bowel. The NHS says more than 400mg in a short time can cause it |
| Stomach cramps or bloating | Common at higher doses | The same laxative effect |
| Nausea | Occasional | Often when taken on an empty stomach |
| Drowsiness | Some people | Usually welcome in the evening; take it at night if it makes you sleepy |
| Skin stinging | With sprays and oils | The salty solution irritates skin. See magnesium spray |
Rare but serious: very high magnesium levels in the blood, mostly in people with kidney failure or who take large amounts of laxatives or antacids containing magnesium. Signs include weakness, very low blood pressure, confusion, a slow or irregular heartbeat and difficulty breathing. Get urgent medical help if this happens. More on this below.
Can magnesium glycinate cause diarrhoea?
Yes, it can, but it is less likely to than magnesium oxide or citrate. Any magnesium can loosen stools if you take more than your gut absorbs at once. With glycinate this usually only happens at higher doses, when it is taken on an empty stomach, or when a product also contains oxide or citrate.
Why glycinate tends to be gentler: magnesium bound to glycine appears to be partly absorbed in a different way, which may leave less in the bowel to draw in water. In a small trial of 12 people with bowel surgery, magnesium glycinate was better tolerated than magnesium oxide (see Sources). It is a small study, so treat it as a pointer rather than proof.
If glycinate gives you loose stools:
- Cut the dose by about half for a week, then build up slowly.
- Take it with food, or split it into two smaller doses.
- Check the label for other forms. Many “glycinate” products are blends, and citrate or oxide in the mix adds a laxative effect. Our own complex combines bisglycinate with citrate, so start with a smaller serving if your stomach is sensitive.
- Add up all your magnesium, including multivitamins, antacids and laxatives.
- Drink plenty of fluids. The NHS says diarrhoea usually stops within 5 to 7 days; call 111 if it lasts longer than 7 days.
For more on this form, see our guide to magnesium glycinate and how it compares in glycinate vs citrate vs threonate.
Which magnesium is gentlest on the stomach?
| Form | Stomach side effects | Often used for |
|---|---|---|
| Glycinate (bisglycinate) | Usually gentle | Evening routines, sleep. See magnesium glycinate |
| Citrate | Mild laxative effect at higher doses | General use, constipation. See magnesium for constipation |
| Oxide | Most likely to cause loose stools | Cheap tablets, constipation |
| Sulphate (Epsom salts) | Strong laxative if swallowed | Baths |
| Threonate, malate, taurate | Generally gentle | Various |
See types of magnesium for a full comparison.
Magnesium spray and oil side effects
The most common side effect of magnesium spray or “magnesium oil” is skin tingling, stinging or itching, sometimes with redness. These products are a strong solution of magnesium chloride in water, which is very salty, so they can sting on dry, freshly shaved or broken skin. Sleep sprays work the same way, so the same applies.
Stomach side effects such as diarrhoea are unlikely from a spray. A 2017 review concluded that the claims for magnesium being absorbed through the skin are scientifically unsupported, so very little, if any, is likely to reach your bloodstream this way (see Sources).
- Patch test first on a small area of skin.
- Avoid broken, irritated or just shaved skin.
- Dilute it or rinse it off after 20 to 30 minutes if it tingles, or switch to a lotion.
- Stop using it if you get a rash, swelling or itching that doesn’t settle.
Read more in magnesium spray: does it work? and our guide to magnesium flakes and baths.
Reaction to a magnesium supplement: allergy signs and when to get help
Most reactions to magnesium supplements are stomach side effects, not allergies. Magnesium is an essential mineral that is in your body and food all the time, so a true allergy to magnesium itself would be unusual. If you do react, it is more likely to be to another ingredient in the product, such as a flavouring, colouring, capsule shell or herbal extract, so check the full ingredients list.
Mild reaction (an itchy rash or hives, mild swelling or stomach upset): stop taking the product and speak to a pharmacist, or call 111 if you are unsure.
Call 999 straight away if you have signs of anaphylaxis, a severe allergic reaction. The NHS lists these as:
- lips, mouth, throat or tongue suddenly becoming swollen
- breathing very fast or struggling to breathe
- a tight throat or struggling to swallow
- skin, tongue or lips turning blue, grey or pale
- suddenly becoming very confused, drowsy or dizzy, or fainting
Also get urgent help for a slow or irregular heartbeat, severe weakness or trouble breathing after taking a lot of magnesium, as these can be signs of high blood magnesium rather than allergy.
Magnesium complexes and calcium magnesium supplements
Magnesium complexes combine magnesium with other ingredients, so side effects can come from those too. Evening blends often include calming herbs or amino acids, which can add to drowsiness. Our Magnesium Glycinate Sleep Complex, for example, contains magnesium bisglycinate and citrate with glycine, L-theanine, chamomile, passionflower, hops and oat straw. If you take medicines that make you drowsy, or any regular medicine, ask your pharmacist first. See what a magnesium complex is.
Calcium and magnesium together is generally fine, but both count towards your limits. The NHS says taking more than 1,500mg of calcium a day could lead to stomach pain and diarrhoea, and that 1,500mg or less is unlikely to cause harm. Calcium and magnesium can also affect how some medicines are absorbed. See taking magnesium and calcium together.
Thinking about calcium as well? Our guide to calcium supplements explains who may need one and how much is too much.
How much magnesium is too much?
- Daily needs: the NHS says adults need 300mg (men) or 270mg (women) a day, and most can get this from food.
- Supplement limit: the NHS says 400mg a day or less from supplements is unlikely to cause any harm.
- Count everything: multivitamins, sleep blends, antacids and laxatives can all contain magnesium.
Magnesium from food doesn’t cause these problems, because the body controls how much it absorbs. For doses by age, form and goal, see how much magnesium per day, and for signs you might be low, see signs of magnesium deficiency.
High blood magnesium (hypermagnesaemia) and kidney disease
Hypermagnesaemia means too much magnesium in the blood. It is uncommon but can be serious. Healthy kidneys remove extra magnesium, which is why the usual result of taking too much is diarrhoea rather than a build-up in the blood.
The risk rises if your kidneys can’t clear magnesium properly. UK government guidance notes that people with kidney disease may be more sensitive to large amounts of magnesium for this reason. A 2023 clinical review lists the main at-risk groups as people with reduced kidney function and people taking magnesium-containing medicines or supplements, often in large amounts such as laxatives or antacids.
- Signs: weakness, nausea, very low blood pressure, confusion or drowsiness, a slow or irregular heartbeat and slow breathing.
- What doctors do: stop the magnesium, and in hospital may give fluids, or dialysis in severe cases.
- If you have kidney disease: don’t take a magnesium supplement unless your kidney team or GP has advised it.
Call 999 or go to A&E for chest symptoms, trouble breathing or fainting. See also how long magnesium stays in your body.
What is the normal range for magnesium in a blood test?
In the UK, the normal range for magnesium in a blood test is usually 0.7 to 1.0 mmol/L for adults. This is the range recommended for NHS labs by the Pathology Harmonisation Reference Group. Some labs use a slightly different range, so check the one printed next to your result.
| Result (mmol/L) | What it means |
|---|---|
| Below 0.7 | Lower than usual. Your GP will look at why |
| 0.7 to 1.0 | Within the usual UK range |
| Above 1.0 | Higher than usual. Your GP will look at why |
- In mg/dL (used in the US): 0.7 to 1.0 mmol/L is about 1.7 to 2.4 mg/dL.
- It is a GP test. Magnesium is measured from a normal blood sample. Ask your GP if you have symptoms or take medicines that affect magnesium.
- A slightly high result can be the sample. Red blood cells contain more magnesium, so if cells break open in the tube the reading can come out high, and the lab may ask for a repeat.
A result is only one part of the picture. Your GP will read it alongside your symptoms and other tests.
Magnesium and medicines
| Medicine | What to do |
|---|---|
| Some antibiotics (tetracyclines such as doxycycline, and quinolones such as ciprofloxacin) | Magnesium can stop them being absorbed properly. Take them at different times; ask your pharmacist how far apart |
| Osteoporosis medicines (bisphosphonates such as alendronic acid) | Can reduce absorption. Take magnesium at a different time of day; ask your pharmacist |
| Thyroid tablets (levothyroxine) | Can reduce absorption. Take them at different times; ask your pharmacist how far apart |
| Some diuretics (water tablets) | Can raise or lower magnesium levels; check with your GP |
| Long-term acid-reducing medicines (such as omeprazole) | Can lower magnesium; your GP may check your levels |
If you take any regular medicine, check with your pharmacist before starting a magnesium supplement.
Who should avoid magnesium supplements?
- people with kidney disease, unless a doctor advises it
- people with a slow heartbeat or heart block
- people with myasthenia gravis
- anyone taking the medicines above, without checking timings first
Our products are for adults only, and we don’t recommend them during pregnancy or breastfeeding. See magnesium in pregnancy and magnesium for kids.
How to reduce side effects
- Choose a gentle form, such as glycinate.
- Start low and build up over a week or two.
- Take it with food if it upsets your stomach.
- Split the dose if you take a larger amount.
- Stay within 400mg a day from supplements.
Our Magnesium Glycinate Sleep Complex provides 148mg of elemental magnesium per serving from bisglycinate and citrate, well within the NHS limit. Magnesium contributes to the reduction of tiredness and fatigue and to normal psychological function. See the best time to take magnesium.
Related guides
- See how to choose the best magnesium supplement in the UK.
- Magnesium citrate side effects.
- Magnesium malate side effects and who should check first.
- Magnesium taurate side effects and who should check first.
- Magnesium oxide side effects and who should check first.
- Can magnesium make you tired or drowsy?
- Magnesium and calcium supplement safety, including medicines to space apart.
- Taking zinc alongside magnesium? Read taking magnesium and zinc together for safe daily limits.
- Powder vs tablets: which is gentler to take.
- Giving magnesium to a child? See magnesium gummies for kids: safe doses by age.
- Magnesium L-threonate side effects and who should avoid it.
- Taking a zinc and copper supplement as well? Read zinc and copper first.
- Magnesium from food doesn’t cause these problems, so see the best food sources of magnesium first.
- If you take magnesium in the evening, our guide to the best magnesium for sleep compares how gentle each form is on the gut at bedtime.
- Trip Mindful Blend side effects for magnesium drinks.
- Taking a branded tablet? See Vitabiotics Ultra Magnesium: form, sleep and side effects.
- Taking a branded capsule? See Wild Nutrition magnesium side effects and directions.
- Before raising your dose, check how long magnesium takes to work; most effects take weeks.
- Magnesium orotate also raises questions about orotic acid; see magnesium orotate side effects and safety.
- For one brand’s advice on the tingle and how many sprays to start with, see BetterYou magnesium.
- Drink powders count too: one serving of Natural Vitality Calm gives 350mg of magnesium, so add up everything you take.
Alongside magnesium, some people like a calm evening ritual. Our Reishi Mushroom Adaptogenic Complex is a reishi hot chocolate made with cacao, so it contains a little natural caffeine. Our Sleep CBD Oil combines broad spectrum CBD with passion flower. The FSA’s 10mg a day figure is precautionary advice rather than a legal limit, set while longer-term safety research continues. At Evopure we think it is a very cautious figure, and many adults use more. If you go above it, increase slowly, and check with your GP or pharmacist first if you take any medication.
FAQs
What are the side effects of magnesium?
The most common are diarrhoea, stomach cramps and nausea, usually from high doses or forms like oxide. Serious side effects are rare.
Can magnesium cause diarrhoea?
Yes, especially above 400mg a day or with oxide and citrate. Glycinate is usually gentler.
Is it safe to take magnesium every day?
For most adults, yes, at 400mg a day or less from supplements. Check with your GP if you have kidney disease or take regular medicines.
Who should not take magnesium?
People with kidney disease, a slow heartbeat or myasthenia gravis, unless a doctor advises it. Ask a pharmacist about timing if you take certain antibiotics, thyroid or osteoporosis medicines.
Which magnesium has the fewest side effects?
Magnesium glycinate (bisglycinate) is usually the gentlest on the stomach.
How long do magnesium side effects last?
Stomach side effects usually ease once you lower the dose or stop. Drink plenty of fluids. The NHS says diarrhoea usually stops within 5 to 7 days; call 111 if it lasts longer than 7 days.
Can you be allergic to magnesium supplements?
A true allergy to magnesium itself is unusual. Reactions are more often to another ingredient, such as a flavouring or herbal extract. Stop the product if you get a rash, and call 999 for swelling of the lips, mouth or throat, or trouble breathing.
What is a normal magnesium level in the UK?
Most UK labs use 0.7 to 1.0 mmol/L for adults. Your GP can arrange the blood test and will explain your result, as some labs use a slightly different range.
Sources
- NHS. Vitamins and minerals: others (magnesium).
- NHS. Vitamins and minerals: calcium.
- NHS. Diarrhoea and vomiting.
- NHS. Anaphylaxis.
- UK Health Security Agency. Magnesium: general information.
- York and Scarborough Teaching Hospitals NHS Foundation Trust. Laboratory test directory: magnesium.
- North Bristol NHS Trust, Severn Pathology. Test information: magnesium.
- Aal-Hamad AH et al. Hypermagnesemia in clinical practice. Medicina (Kaunas), 2023. PMID 37512002.
- Gröber U et al. Myth or reality: transdermal magnesium? Nutrients, 2017. PMID 28788060.
- Schuette SA et al. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN, 1994. PMID 7815675.
- NHS. Levothyroxine.
- NHS. Doxycycline.
- NHS. Alendronic acid.
This guide is general information, not medical advice. Evopure makes the Magnesium Glycinate Sleep Complex.
