Magnesium glycinate vs citrate vs oxide: which form actually matters?
For sleep, magnesium bisglycinate is the sensible default: it is gentle on the gut at a meaningful dose, it brings glycine along with it, and as of 2025 it has a placebo-controlled trial of its own. Citrate is a fair, cheaper alternative if your bowels cope. Oxide is poorly absorbed, yet it is the form used in the best-known positive trial, which should make you suspicious of anyone who calls it useless. And the thing the label usually hides, elemental versus compound weight, matters more than the form debate.
Why is there more than one kind of magnesium?
Because magnesium cannot exist on its own in a capsule. It has to be bound to something, and the partner determines how much actual magnesium you get, how well it dissolves, what it does to your digestion, and what it costs. Inorganic salts pair it with oxygen, chloride or sulphate. Organic forms pair it with an acid such as citrate or malate, or with an amino acid such as glycine or taurine, or with threonic acid in the case of threonate. Each partner has a weight of its own, and that is where labels get slippery.
What does "500mg of magnesium" on a label actually mean?
Often far less than it sounds. Because the partner molecule has weight, only a fraction of any compound is magnesium itself. Magnesium oxide is roughly 60 percent magnesium by weight. Citrate is about 16 percent. Bisglycinate, where each magnesium carries two glycine molecules, is around 14 percent. So a capsule declaring "500mg magnesium bisglycinate" may contain about 70mg of magnesium, while "500mg magnesium oxide" contains around 300mg.
The honest figure is the elemental dose, the amount of magnesium you actually receive, and a good label states it plainly. Watch also for "buffered" bisglycinate, which is bisglycinate blended with oxide to raise the elemental percentage and lower the price. If the label does not say "fully reacted", assume it is not.
Which form is absorbed best?
The evidence favours the organic forms over oxide, with less daylight between the organic forms than the marketing suggests.
In a classic comparison of commercial preparations, magnesium oxide was markedly less bioavailable than the organic salts [1]. A 60-day randomised trial found citrate more bioavailable than oxide, and also more than an amino-acid chelate [2], which is worth remembering before paying a premium for any chelate on absorption grounds alone. The study most often quoted for bisglycinate's superiority is more modest than its reputation: in twelve patients with part of the small bowel removed, absorption from bisglycinate and oxide was similar across the group, bisglycinate was clearly better only in the four with the worst absorption, and it was better tolerated by everyone [3].
That last word is the honest summary. Bisglycinate's proven advantage is tolerability. Its absorption advantage over oxide is real but situational, and over citrate it is not established.
Which form has evidence for sleep?
Here the story turns on itself. The most-cited positive trial, 46 older adults with insomnia given 500mg of elemental magnesium daily for eight weeks, with improvements in insomnia severity, sleep efficiency and time to fall asleep, used magnesium oxide [4]. The form now dismissed in every comparison article is the form that produced the flagship result. A meta-analysis of trials in older adults found magnesium shortened sleep onset by around 17 minutes, with certainty of evidence graded low to very low [5].
Bisglycinate acquired a trial of its own in 2025. In 155 adults aged 18 to 65 with poor sleep, 250mg of elemental magnesium as bisglycinate for four weeks reduced insomnia severity more than placebo, by 3.9 points against 2.3. The effect was statistically significant and small, other outcomes such as fatigue and mood did not differ, and the benefit looked larger in people whose diets were low in magnesium to begin with [6]. The study was funded by the university that ran it. That is a modest, believable result, and the first to test the form most people now buy.
Threonate is sold on brain research, and it now has a human sleep trial, though the picture is thin. A three-week trial gave around 80 adults one gram of magnesium threonate a day, roughly 70mg of elemental magnesium, and reported improved sleep on questionnaires with little change on wearable sleep tracking. It was funded by the ingredient's distributor [7]. If cognition is your interest, threonate has a case. For sleep it has the weakest elemental dose per pound spent of any form here.
We could find no sleep trial of citrate specifically. And magnesium sprays, oils and bath flakes rest on skin absorption that a 2017 review found has never been convincingly demonstrated [8].
Which form is easiest on the gut?
This is where the forms genuinely part company. Magnesium draws water into the bowel, which is why citrate is sold as a laxative in its own right and why oxide at a useful dose sends a fair proportion of people to the bathroom. Bisglycinate is consistently the gentlest, which was the clearest finding of the bowel-resection study [3]. NHS guidance notes that up to 400mg a day from supplements is unlikely to cause harm, and that higher amounts can cause diarrhoea [9].
So which should you choose?
If you want magnesium for sleep and a comfortable gut, choose fully reacted bisglycinate with the elemental dose stated, somewhere in the 200 to 300mg range that sits under the NHS ceiling and near the trial doses. If you tend towards constipation, citrate's side effect is a feature. If budget rules, oxide has produced a positive trial at a higher dose, and you should expect it to move your bowels. Threonate is a cognition product with a sleep claim attached. Sprays are a bath.
The UK reference intake is about 300mg a day for men and 270mg for women, and intakes below that are common [9], so a supplement is topping up a shortfall for many people as much as adding anything new.
What we chose, and why
Stay Asleep uses fully reacted magnesium bisglycinate at 300mg of elemental magnesium, and the choice was tolerability first. A dose that matters has to be a dose you can take every night without consequences, and bisglycinate is the form that allows that. It also arrives with over a gram and a half of glycine attached, an amino acid with sleep evidence of its own, which we have reviewed separately. For the broader question of whether magnesium helps sleep at all, our honest look at the evidence is the place to start, and if your nights break in the small hours, our article on why you wake up at 3am covers what is happening in that half of the night.
Common questions
Is magnesium glycinate better than citrate for sleep?
For tolerability, yes: bisglycinate is gentler on the gut at the doses that matter [3]. For absorption, the one head-to-head that included a chelate favoured citrate [2]. For sleep specifically, bisglycinate now has a small positive trial [6] and citrate has none we could find, so the honest answer is that glycinate is better supported and better tolerated, and citrate is a reasonable cheaper choice if it suits you.
How much elemental magnesium should I take for sleep?
The trials used 250mg to 500mg of elemental magnesium a day [4][6]. NHS guidance puts 400mg a day from supplements as the level unlikely to cause harm [9]. Check that the label states the elemental figure, since a compound weight can overstate the dose several times over.
Does magnesium oxide work at all?
It did in the most-cited trial, at 500mg of elemental magnesium in older adults [4]. It is less well absorbed than organic forms [1][2], so more of it ends up in the bowel, which is why it tends to be laxative at useful doses.
Is magnesium threonate worth the price?
For cognition it has the more interesting evidence. For sleep, the trial is short, funded by the ingredient's distributor, and mainly shows questionnaire improvements [7], and each gram delivers only around 70mg of elemental magnesium. Spend elsewhere if sleep is the goal.
References
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnesium Research. 2001;14(4):257-262.
- Walker AF, Marakis G, Christie S, Byng M. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnesium Research. 2003;16(3):183-191.
- Schuette SA, Lashner BA, Janghorbani M. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. Journal of Parenteral and Enteral Nutrition. 1994;18(5):430-435.
- Abbasi B, Kimiagar M, Sadeghniiat K, Shirazi MM, Hedayati M, Rashidkhani B. The effect of magnesium supplementation on primary insomnia in elderly: a double-blind placebo-controlled clinical trial. Journal of Research in Medical Sciences. 2012;17(12):1161-1169.
- Mah J, Pitre T. Oral magnesium supplementation for insomnia in older adults: a systematic review and meta-analysis. BMC Complementary Medicine and Therapies. 2021;21:125.
- Schuster J, Cycelskij I, Lopresti A, Hahn A. Magnesium bisglycinate supplementation in healthy adults reporting poor sleep: a randomized, placebo-controlled trial. Nature and Science of Sleep. 2025;17:2027-2040.
- Hausenblas HA, Lynch T, Hooper S, Shrestha A, Rosendale D, Gu J. Magnesium-L-threonate improves sleep quality and daytime functioning in adults with self-reported sleep problems: a randomized controlled trial. Sleep Medicine: X. 2024;8:100121.
- Gröber U, Werner T, Vormann J, Kisters K. Myth or reality: transdermal magnesium? Nutrients. 2017;9(8):813.
- NHS. Vitamins and minerals: others (magnesium). nhs.uk.