Magnesium is an essential mineral supporting over 300 bodily functions, from energy production to heart health. Whilst most UK adults obtain adequate magnesium through diet, supplements are increasingly popular. However, excessive magnesium intake—particularly from supplements—can cause significant health problems, ranging from gastrointestinal distress to serious cardiac complications. Understanding how much magnesium is too much is crucial for safe supplementation. UK health authorities have established clear upper limits: 400 mg daily from supplements is considered safe for most adults with normal kidney function. This article explores recommended intakes, signs of excess, health risks, and when to seek medical advice.

Recommended Daily Magnesium Intake in the UK

Magnesium is an essential mineral that plays a vital role in over 300 enzymatic reactions throughout the body, including energy production, protein synthesis, muscle and nerve function, and bone health. The UK's Department of Health and Social Care has established reference nutrient intakes (RNIs) to guide adequate daily consumption.

For adults, the recommended daily intake varies by age and sex:

• Men aged 19 and over: 300 mg per day

• Women aged 19 and over: 270 mg per day

• For pregnant women, no specific increased requirement is recommended in UK guidance; discuss any supplements with your midwife or GP

• Adult requirements remain consistent across age groups

Most people in the UK can meet these requirements through a balanced diet rich in magnesium-containing foods. Excellent dietary sources include:

• Green leafy vegetables (spinach, kale)

• Nuts and seeds (almonds, pumpkin seeds, cashews)

• Whole grains (brown rice, wholemeal bread)

• Legumes (black beans, chickpeas)

• Dark chocolate and cocoa

• Fish (mackerel, salmon)

The National Diet and Nutrition Survey indicates that whilst many UK adults consume adequate magnesium through diet alone, certain groups may have lower intakes. These include older adults, people with gastrointestinal conditions affecting absorption, those with chronic alcohol consumption, and individuals taking certain medications. However, supplementation should only be considered after dietary optimisation and, ideally, following discussion with a healthcare professional to avoid exceeding safe limits.

Signs You're Taking Too Much Magnesium

Excessive magnesium intake, particularly from supplements, can lead to a condition called hypermagnesaemia, characterised by elevated magnesium levels in the blood (typically above 1.0 mmol/L). The body typically regulates magnesium effectively through renal excretion, but when intake overwhelms this mechanism—or when kidney function is impaired—symptoms may develop.

The most common early signs of magnesium excess include:

• Gastrointestinal disturbances: Diarrhoea is often the first and most prominent symptom, as unabsorbed magnesium in the intestines draws water into the bowel. This osmotic effect can cause loose stools, abdominal cramping, and nausea.

• Nausea and vomiting: These symptoms may occur alongside or independently of diarrhoea, particularly with high-dose supplementation.

• Abdominal discomfort: Bloating and generalised abdominal pain may develop as the digestive system responds to excess magnesium.

As magnesium levels continue to rise, more serious symptoms may emerge:

• Muscle weakness and lethargy

• Confusion or altered mental state

• Irregular heartbeat (arrhythmias)

• Difficulty breathing

• Hypotension (low blood pressure)

• In severe cases, cardiac arrest

It is important to note that magnesium toxicity from dietary sources alone is extremely rare in individuals with normal kidney function, as the kidneys efficiently eliminate excess amounts. Toxicity almost always results from excessive supplementation, particularly with high-dose magnesium salts, magnesium-containing laxatives or antacids, or occurs in people with compromised renal function. If you experience persistent gastrointestinal symptoms after starting magnesium supplements, discontinue use and consult your GP or call NHS 111 for advice.

Health Risks of Excessive Magnesium Supplementation

Whilst magnesium is essential for health, excessive supplementation carries several potential risks, particularly for vulnerable populations. Understanding these risks is crucial for safe supplementation practices.

Cardiovascular complications represent one of the most serious concerns with magnesium excess. Hypermagnesaemia can interfere with normal cardiac conduction, potentially causing:

• Bradycardia (abnormally slow heart rate)

• Heart block and other conduction abnormalities

• Hypotension, which may cause dizziness and fainting

• In extreme cases, cardiac arrest

These effects occur because magnesium acts as a natural calcium channel blocker, and excessive levels can over-suppress cardiac electrical activity.

Neuromuscular effects develop as magnesium interferes with nerve signal transmission and muscle contraction. Patients may experience progressive muscle weakness, reduced reflexes, and in severe cases, respiratory muscle weakness requiring medical intervention. The central nervous system may also be affected, leading to drowsiness, confusion, and altered consciousness.

Renal considerations are particularly important. Individuals with chronic kidney disease (CKD) have reduced capacity to excrete magnesium and are at substantially higher risk of toxicity, even with modest supplementation. Magnesium supplements are contraindicated in severe renal impairment and should be used with caution in moderate impairment, as noted in product SmPCs (Summaries of Product Characteristics).

Drug interactions pose additional risks. Magnesium can interact with several medications, including:

• Bisphosphonates (used for osteoporosis): Take magnesium at least 2 hours after these medications

• Certain antibiotics (tetracyclines, quinolones): Separate by at least 2 hours before or 4-6 hours after antibiotics

• Levothyroxine: Take magnesium supplements at least 4 hours apart

• Diuretics: Loop and thiazide diuretics may increase magnesium loss, while potassium-sparing diuretics may increase levels

It's also worth noting that long-term use of proton pump inhibitors (PPIs) may cause hypomagnesaemia (low magnesium levels), rather than excess, and may require monitoring in some patients.

Patients taking these medications should discuss magnesium supplementation with their GP or pharmacist to avoid potentially harmful interactions.

Safe Upper Limits for Magnesium Supplements

Health authorities have established safe upper limits specifically for magnesium obtained from supplements and fortified foods, distinct from dietary intake.

The European Food Safety Authority (EFSA) has set a tolerable upper intake level for supplemental magnesium at 250 mg per day for adults, including during pregnancy and lactation.

In the UK, the Expert Group on Vitamins and Minerals (EVM) has established a guidance level of 400 mg per day from supplements only. The NHS advises that taking 400 mg or less of magnesium supplements a day is unlikely to cause harm.

These limits apply to magnesium from supplements and fortified foods only—not from natural food sources. This distinction is important because magnesium from diet is considered safe even at higher intakes, as the body regulates absorption and excretion efficiently when magnesium comes from food.

These thresholds were established based on evidence that higher supplemental doses commonly cause gastrointestinal effects, particularly osmotic diarrhoea. The limits provide a margin of safety for the general population with normal kidney function.

Different magnesium compounds vary in their elemental magnesium content and bioavailability:

• Magnesium oxide: Contains high elemental magnesium (~60%) but has lower bioavailability; commonly causes gastrointestinal effects

• Magnesium citrate: Moderate elemental content (~16%) with good bioavailability; better tolerated

• Magnesium glycinate: Lower elemental content but generally well tolerated

• Magnesium chloride: Used in some oral preparations and topical products

When selecting supplements, check the label for elemental magnesium content rather than the total compound weight. A 500 mg magnesium oxide tablet, for example, contains approximately 300 mg of elemental magnesium.

Regarding topical magnesium products (oils, sprays, bath salts), it's important to note that evidence for meaningful systemic absorption through the skin is limited, and these products are not licensed for treating magnesium deficiency.

Special populations require particular caution:

• Individuals with renal impairment should avoid magnesium supplements unless specifically prescribed and monitored by a physician

• Older adults may have reduced kidney function and should use lower doses

• Pregnant and breastfeeding women should not exceed recommended amounts without medical supervision

NICE guidance emphasises that supplementation should be evidence-based and targeted to those with demonstrated deficiency or specific clinical indications, rather than routine use in the general population.

When to Seek Medical Advice About Magnesium Levels

Knowing when to consult a healthcare professional about magnesium intake is essential for preventing complications and ensuring appropriate management.

Seek urgent medical attention (call 999 or attend A&E) if you experience:

• Severe muscle weakness or paralysis

• Difficulty breathing or shortness of breath

• Severe palpitations with dizziness or fainting

• Chest pain

• Severe confusion or loss of consciousness

• Persistent vomiting preventing fluid intake

These symptoms may indicate severe hypermagnesaemia requiring immediate intervention, including intravenous calcium gluconate and potentially dialysis in extreme cases.

Contact NHS 111 or your GP promptly if you develop:

• Persistent diarrhoea lasting more than 48 hours after starting magnesium supplements

• Ongoing nausea or abdominal cramping

• Unusual fatigue or muscle weakness

• Dizziness or light-headedness

• Mild palpitations without other concerning symptoms

• Any concerning symptoms after beginning supplementation

You should discuss magnesium supplementation with your GP before starting if you:

• Have chronic kidney disease or any history of renal impairment

• Take regular medications, particularly those listed in the interactions section

• Have a history of heart rhythm disorders

• Are pregnant or breastfeeding

• Have a diagnosed magnesium deficiency requiring treatment

Routine monitoring of magnesium levels through blood tests is not typically necessary for healthy individuals taking supplements within safe limits. However, your GP may arrange serum magnesium testing if you have risk factors for deficiency or toxicity, symptoms suggestive of abnormal levels, or conditions affecting magnesium metabolism.

If you're considering magnesium supplementation, discuss your diet and symptoms with your GP or a registered dietitian first. They can assess whether you genuinely need supplementation or whether dietary modifications might be more appropriate and safer. Remember that more is not always better—meeting your body's requirements through a balanced diet remains the gold standard for nutritional health.

If you experience any suspected side effects from magnesium supplements, you can report them through the MHRA Yellow Card Scheme, which helps monitor the safety of supplements and medicines.

Frequently Asked Questions