What's actually in a magnesium supplement: glycinate, citrate, chloride, and what they each do.
Magnesium glycinate, magnesium citrate, magnesium chloride. The label says one thing. The molecule does another.
Walk into any pharmacy and you'll see at least five bottles labeled magnesium. They are not the same thing. Different forms of the mineral are paired with different carrier compounds, and the carrier is what determines whether the magnesium ends up in your muscles, your gut, your skin, or none of the above.
The category isn't one thing
Magnesium itself is just an element. To put it into a pill or a cream, manufacturers have to bond it to something else. That something else, the carrier, decides what the supplement actually does. Glycinate makes magnesium gentle on the stomach and tends to push it toward the nervous system. Citrate moves things through the digestive tract. Chloride is the form most readily absorbed through the skin.
Glycinate: the sleep-and-anxiety one
Magnesium glycinate is magnesium bonded to glycine, an amino acid that on its own has a mild calming effect. It's the form most often recommended for people who want to support sleep, ease muscle tension, or take magnesium without the gastric distress that comes with cheaper forms. Absorption is high, side effects are low, and it tends to be the one that registered dietitians reach for first.
Citrate: the one that moves things through
Magnesium citrate is bonded to citric acid. It's well-absorbed but it also has a real osmotic effect on the gut, which is why it's a common ingredient in over-the-counter laxatives. If you take it for sleep and end up running to the bathroom at 2am, that's why. For occasional constipation it's useful. For long-term magnesium repletion it's not the form most people want.
Chloride: the topical one
Magnesium chloride is the form most often used in topical preparations. The chloride ion is small and the bond is loose, so the magnesium is released easily through the skin. A 2023 pilot study published in the Journal of Palliative Medicine found that participants applying topical magnesium chloride daily for twelve weeks saw measurable reductions in neuropathic symptom scores while their blood magnesium levels barely moved, suggesting the magnesium was working locally rather than systemically.
Oxide: technically magnesium, mostly not absorbed
Magnesium oxide is the cheapest form on the shelf and the one most likely to appear in mass-market multivitamins. Bioavailability is poor: studies put absorption around 4% versus 23-30% for citrate and 24% for glycinate. If a label tells you a multivitamin contains 250mg of magnesium and the form is oxide, only a small fraction is reaching your bloodstream.
Which form matters depends on what you're trying to do
For most people the question isn't do I need magnesium but which form for which job. Glycinate for sleep and anxiety. Citrate for occasional constipation. Chloride applied locally for tissue-level effects, as a complement rather than a replacement for systemic magnesium. Oxide is rarely the right answer outside of a basic multivitamin.
This is the kind of distinction the supplement aisle doesn't help with. The label says magnesium. What's behind it can be five different things doing five different things, and figuring out which one you actually need usually requires reading more than the front of the bottle.