Magnesium
mineral · also known as Mg
This is general information, not medical advice. Supplements can interact with medicines and health conditions. Always check with your doctor or pharmacist before starting or changing supplements.
What it is
Magnesium is an essential mineral involved in several hundred enzymatic reactions in the body, including energy production, muscle and nerve function, blood glucose regulation, and blood pressure control. Many adults in high-income countries consume less magnesium than recommended, and mild deficiency is more common in people with poor diets, gastrointestinal disease, or those taking certain medications such as proton pump inhibitors or diuretics. People take magnesium supplements to correct low intake, ease muscle cramps, support sleep, or as an adjunct for migraine prevention or blood pressure control. Different salts (glycinate, citrate, oxide) vary widely in how much elemental magnesium they provide and how well that magnesium is absorbed. Most healthy adults tolerate supplemental magnesium well; the main limiting side effect at higher doses is diarrhoea, which is both dose- and form-dependent.
| Form | Elemental fraction | Notes |
|---|---|---|
| Magnesium glycinate | 14.1% | Magnesium bound to the amino acid glycine; well absorbed and generally better tolerated (less GI upset) than magnesium oxide, which is why it is often chosen for sleep- or relaxation-focused use. |
| Magnesium citrate | 16.1% | Magnesium bound to citric acid; well absorbed relative to oxide, with a mild-to-moderate laxative effect that becomes more noticeable at higher doses. |
| Magnesium oxide | 60.3% | Highest elemental magnesium content by weight but poorly absorbed (studies have found single-digit to low-double-digit percent absorption); most useful as a low-cost osmotic laxative or antacid rather than for correcting magnesium deficiency. |
Evidence-based benefits
- Grade B· Limited evidence[1]
Oral magnesium supplementation may modestly reduce migraine attack frequency when used preventively, though the supporting trial evidence is inconsistent across formulations and doses.
An evidence-based review of oral magnesium trials for migraine prevention (mainly citrate or oxide, roughly 400-600 mg/day) found some trials reported fewer migraine attacks versus placebo, but rated the overall evidence as limited due to small sample sizes and heterogeneous dosing and formulations.
- Grade A· Strong evidence[1]
Magnesium supplementation modestly lowers blood pressure on average, with the largest reductions in people who are hypertensive or magnesium-deficient; little to no effect is seen in normotensive people.
A 2025 systematic review and meta-analysis of 38 randomised controlled trials (2,709 participants, median dose 365 mg elemental magnesium/day for 12 weeks) found average reductions of 2.8 mmHg systolic and 2.1 mmHg diastolic blood pressure versus placebo. Reductions were larger (roughly 6-8 mmHg systolic) in people with hypertension or hypomagnesemia, but not statistically significant in normotensive groups.
- Grade B· Limited evidence[1]
Magnesium glycinate supplementation may modestly improve subjective sleep quality in adults who report poor sleep, though the effect size is small.
A 2025 randomised, placebo-controlled trial in 155 adults with self-reported poor sleep found that 250 mg/day elemental magnesium (as bisglycinate) produced a small but statistically significant improvement in Insomnia Severity Index scores versus placebo after 4 weeks (Cohen's d approximately 0.2), with larger gains in those with lower baseline dietary magnesium intake.
- Grade B· Limited evidence[1]
Pooled randomised-trial evidence does not show a significant benefit of magnesium supplementation on glycemic control (fasting blood glucose or HbA1c) in people with type 2 diabetes; a trend toward improvement exists but does not reach statistical significance, even in those with hypomagnesemia.
A meta-analysis of randomised controlled trials of magnesium supplementation in type 2 diabetes found a non-significant trend toward improved glycemic control in the magnesium-treated group (mean difference -0.19, 95% CI -0.58 to 0.21), with a stronger but still non-significant trend in patients with hypomagnesemia (mean difference -1.16, 95% CI -2.92 to 0.6). The authors concluded that routine magnesium supplementation for glycemic control in type 2 diabetes cannot be recommended based on this evidence.
Risks & contraindications
- Grade B· Limited evidence[1]
Magnesium oxide, and other magnesium salts at higher doses, commonly cause diarrhoea and gastrointestinal upset from their osmotic laxative effect in the gut.
A 2023 review of intervention studies reporting diarrhoea as an adverse event found variable rates depending on the magnesium salt and dose; seven studies spanning 128-1200 mg/day found no significant difference in diarrhoea versus control, while other trials (particularly with magnesium oxide) reported diarrhoea in close to half of participants, reflecting oxide's very poor absorption.
- Grade C· Preliminary evidence[1]
In people with reduced kidney function, magnesium supplementation carries a risk of hypermagnesemia, which can cause neuromuscular, cardiac, and respiratory complications.
A 2023 narrative review found hypermagnesemia is uncommon in people with normal kidney function but occurs mainly in those with impaired renal clearance or high combined intake from supplements, laxatives, or antacids; reported effects range from muscle weakness to respiratory failure and coma in severe cases.
Dosage & how to take it
- Magnesium glycinate: 200–400 mg/day, in the evening, with or without food. Dose is elemental magnesium. Glycinate's gentler GI profile makes it a common choice for evening/sleep-support use.
- Magnesium citrate: 200–400 mg/day, in the evening, with or without food. Dose is elemental magnesium. May loosen stool at the higher end of the range; splitting the dose can reduce GI effects.
- Magnesium oxide: 200–400 mg/day, in the evening, with or without food. Dose is elemental magnesium. Poor absorption means a larger share acts as an osmotic laxative in the gut; not the preferred form for correcting deficiency.
Interactions
Magnesium is a polyvalent cation, and levothyroxine can bind to polyvalent cations in the gut, forming a complex that your body absorbs poorly, which can lower the medication's effectiveness. A 2025 randomised crossover trial in healthy adults tested this directly: taking magnesium aspartate together with a levothyroxine dose significantly reduced how much levothyroxine reached the bloodstream (AUC reduced by 12%, with peak level also reduced and delayed); magnesium citrate showed a smaller, seven percent reduction that did not reach statistical significance in this study. Because the effect was clearest with one magnesium form and only a non-significant trend with another, and because the trial was small (15 healthy subjects, not people with hypothyroidism), it's reasonable to treat any magnesium supplement as a candidate for the same interaction rather than assume citrate is exempt. If you take levothyroxine, take your magnesium supplement at least 4 hours before or after your dose, and mention any new magnesium supplement to your doctor if your thyroid levels have been stable.
Take at least 4h apart from Thyroid hormones.
Our medication coverage is not exhaustive. Absence of a warning here is not evidence of safety. Always confirm with your pharmacist.
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