The Short Version
Magnesium glycinate is the best default choice for most people - high absorption, gentle on the stomach, secondary benefits from the glycine. Magnesium citrate is the budget alternative with similar absorption. Magnesium oxide is the most common form in cheap supplements and the worst absorbed. Magnesium L-threonate (Magtein) is the only form with clinical evidence for cognitive benefits, but the premium price is only justified if cognition is your specific goal.
See our full magnesium glycinate scorecard for product comparisons and cost-per-dose analysis.
Quick Picks
- Best for Sleep and Anxiety: Thorne Magnesium Bisglycinate - third-party tested, 200mg elemental per serving, clean label, ~$0.20/day
- Best Value: NOW Magnesium Citrate - widely available, good absorption, ~$0.07/day at 200mg elemental
- Best Premium (Sleep/Anxiety): Pure Encapsulations Magnesium Glycinate - practitioner-grade quality standards, ~$0.25/day
- Best for Cognitive Function: Magtein (MagMind) - the only clinically studied magnesium L-threonate brand, ~$0.70/day
Why Magnesium Deficiency Is Common and Why It Matters
Approximately 50% of Americans consume less than the estimated average requirement for magnesium, based on the 2005-2006 National Health and Nutrition Examination Survey data analyzed by the NIH. The recommended daily allowance is 310-420mg for adults depending on age and sex. As detailed in a 2017 review in Scientifica, magnesium plays a role in over 300 enzymatic reactions in the body - including ATP production, protein synthesis, muscle and nerve function, blood glucose regulation, and blood pressure regulation.
Subclinical magnesium inadequacy (not low enough to produce overt deficiency symptoms, but below optimal levels) has been associated in observational studies with poor sleep quality, anxiety, muscle cramps, fatigue, and elevated inflammatory markers. Whether supplementation corrects these issues in people with subclinical inadequacy is less well-established than the evidence for treating documented deficiency, but the practical threshold for supplementing is fairly low given magnesium's safety profile and low cost in well-absorbed forms.
The Form Breakdown: Absorption and Best Use Cases
Magnesium Glycinate (Bisglycinate)
Best for: Sleep, anxiety, general supplementation
Elemental magnesium content: Approximately 14% by weight (a 3,500mg capsule of magnesium glycinate compound contains about 200mg elemental magnesium)
Magnesium glycinate binds magnesium to glycine, an amino acid with its own calming properties. A 2015 study in Neuropsychopharmacology showed that 3 grams of glycine before bed improved subjective sleep quality and reduced daytime fatigue. A 2012 double-blind, placebo-controlled trial in elderly subjects with insomnia found that 500mg of elemental magnesium per day significantly improved sleep time, sleep efficiency, and melatonin levels compared to placebo.
Glycinate is well-tolerated with a low rate of GI side effects compared to citrate. It is the most common form recommended by clinicians specifically for sleep and anxiety support. The main practical downside: the large molecule size means capsules are physically large, and you may need 2-4 capsules per serving to reach the effective dose. Powders are a reasonable alternative in this form.
What to look for on the label: The elemental magnesium amount (usually listed as "Magnesium" in the Supplement Facts panel, separate from the compound weight). Target 200-400mg elemental magnesium per day. If a product only lists the total glycinate compound weight and not the elemental amount, that is a transparency red flag.
Magnesium Citrate
Best for: Budget general supplementation, mild constipation relief
Elemental magnesium content: Approximately 16% by weight
Magnesium citrate has high bioavailability - comparable to glycinate and significantly better than oxide. A 2003 study in Magnesium Research directly compared bioavailability across forms and found citrate absorbed substantially better than oxide. It is widely available, inexpensive (typically $0.05-$0.15/day at effective dose), and has been used in digestive health applications for decades.
The notable trade-off: citrate has a laxative effect that becomes more pronounced above 200mg elemental magnesium. For people with constipation, this is a feature. For people who want magnesium without the laxative effect, this is a reason to prefer glycinate. Start at the lower end of the dose range (150-200mg elemental) and adjust from there.
Magnesium Oxide
Best for: Almost nothing - this is the one to avoid
Elemental magnesium content: 60% by weight (the highest of any form, which is why it appears on cheap supplements)
Magnesium oxide looks good on a label because 400mg of magnesium oxide provides 240mg elemental magnesium in a small capsule. The problem is absorption. The same 2003 study found magnesium oxide had approximately 4-5% bioavailability, compared to 25-30% for citrate. A 400mg magnesium oxide capsule may only deliver 10-20mg of absorbed magnesium - less than a cup of spinach.
Magnesium oxide is estimated to account for roughly 60% of the magnesium supplement market by volume, largely because it is the cheapest form to manufacture and looks impressive on a label. If you have been taking magnesium oxide and noticed no effect, the absorption rate is almost certainly the explanation. Switch to citrate or glycinate.
The one legitimate use case: because the unabsorbed magnesium draws water into the colon, magnesium oxide can provide constipation relief. But for this purpose, citrate works better and costs about the same.
Magnesium L-Threonate
Best for: Cognitive function (the only form studied for this specifically)
Elemental magnesium content: Approximately 8% by weight
Magnesium L-threonate was developed by researchers at MIT and is sold under the brand name Magtein. The claim is that the threonate molecule uniquely facilitates magnesium transport across the blood-brain barrier, increasing brain magnesium levels more effectively than other forms. This is based on a 2010 paper in Neuron showing significant cognitive improvements in rats given magnesium L-threonate.
Human evidence exists but is limited. A 2016 randomized controlled trial in adults aged 50-70 with cognitive complaints (published in the Journal of Alzheimer's Disease) found improvements in several cognitive measures over a 12-week period with 2,000mg Magtein per day (providing 144mg elemental magnesium). This is a single trial with a moderate sample size, and no published trial has yet directly compared brain magnesium elevation from threonate vs. other well-absorbed forms in humans.
The cost runs $0.50-$1.00/day. If cognitive function is your primary concern and you have the budget, this is a reasonable choice. If you are supplementing primarily for sleep, anxiety, muscle function, or general deficiency correction, the evidence does not support the premium over glycinate or citrate.
Magnesium Malate
Best for: Possible energy and fatigue support (limited evidence)
Magnesium malate combines magnesium with malic acid, a compound involved in the Krebs cycle (cellular energy production). It has been studied primarily in the context of fibromyalgia. A small 1995 pilot study showed some benefit for fibromyalgia symptoms with magnesium malate supplementation, but this has not been well-replicated. For general magnesium supplementation without a specific cognitive or digestive goal, malate is a reasonable choice with good tolerability, but the energy-specific claims should be viewed skeptically given the evidence base.
Reading the Label: Elemental vs. Compound Weight
This is the single most important thing to understand when buying magnesium. When a label says "Magnesium Glycinate 500mg," that is the weight of the whole magnesium glycinate compound - not the elemental magnesium content. Because glycine is a much larger molecule than magnesium, the elemental magnesium in 500mg of magnesium glycinate is only about 70mg.
A reputable manufacturer will list both numbers in the Supplement Facts panel: the compound dose and the elemental magnesium. Look for the line that says just "Magnesium" with a value in mg - that is the elemental amount, and that is the number to use when calculating your dose. Target 200-400mg of elemental magnesium per day from supplements (separate from dietary intake).
Cost Per Effective Dose: How the Forms Compare
- Magnesium oxide: $0.03-$0.08/day - appears cheapest but delivers the least absorbed magnesium. Poor value despite low price.
- Magnesium citrate: $0.05-$0.15/day - strong absorption, genuine value
- Magnesium glycinate: $0.10-$0.30/day - best overall for most people
- Magnesium malate: $0.10-$0.25/day - similar range to glycinate
- Magnesium L-threonate (Magtein): $0.50-$1.00/day - justified only for cognitive-specific use
Safety and Upper Limits
The tolerable upper intake level for supplemental magnesium is 350mg elemental per day for adults. Exceeding this is generally not dangerous in people with normal kidney function - the kidneys excrete excess magnesium efficiently - but increases the risk of loose stools, cramping, and nausea. People with impaired kidney function should consult their doctor before supplementing, as kidney disease impairs the body's ability to clear excess magnesium and can allow dangerous accumulation.
Magnesium from food is not subject to the upper limit, as food-based magnesium has not been shown to cause adverse effects. The 350mg upper limit applies only to supplemental magnesium.
FAQ
When should I take magnesium?
For sleep and relaxation purposes, 30-60 minutes before bed is a common recommendation and aligns with how most people use glycinate. For general deficiency correction, timing matters less than consistency. Take it with food to reduce GI side effects if needed.
Can magnesium interact with medications?
Yes - magnesium can reduce the absorption of certain antibiotics (quinolones, tetracyclines) and bisphosphonates (osteoporosis medications) when taken at the same time. Separate magnesium from these medications by at least 2 hours. Magnesium can also enhance the effects of blood pressure medications. If you take prescription medications, discuss magnesium supplementation with your pharmacist or doctor.
How long before I notice an effect?
For sleep, some people notice improvement within a few days of starting magnesium glycinate at an effective dose. For addressing deficiency-related symptoms like muscle cramps or fatigue, 2-4 weeks of consistent supplementation is typically needed for tissue levels to normalize.
These statements have not been evaluated by the FDA. Dietary supplements are not intended to diagnose, treat, cure, or prevent any disease.