Magnesium salts are compounds formed when magnesium bonds with another molecule, such as sulfate, chloride, citrate, or oxide. They show up everywhere: dissolved in seawater, pressed into supplement tablets, mixed into food products, and used in hospital settings. The specific partner molecule determines how the salt behaves, how well your body absorbs it, and what it’s useful for.
How Magnesium Salts Form
Magnesium is a highly reactive metal that readily gives up two electrons, making it eager to bond with negatively charged molecules called anions. When it pairs with sulfate, you get magnesium sulfate. With chloride, magnesium chloride. With citric acid, magnesium citrate. Each pairing creates a distinct salt with its own solubility, taste, and behavior in the body.
These salts fall into two broad categories. Inorganic magnesium salts, like magnesium oxide, magnesium sulfate, and magnesium chloride, pair magnesium with simple mineral anions. Organic magnesium salts, like magnesium citrate, magnesium glycinate, and magnesium stearate, pair it with carbon-containing molecules. This distinction matters most when it comes to how easily your digestive system can absorb the magnesium inside.
Where They Occur Naturally
Magnesium salts are abundant in nature, particularly in the ocean. When seawater evaporates, the residue left behind is about 77% sodium chloride (table salt), but roughly 16% is magnesium salts: magnesium chloride makes up nearly 11%, magnesium sulfate about 5%, and magnesium bromide a small fraction. Salt brines, the concentrated solutions left after most of the water evaporates from seawater, are commercially harvested in countries like the United States and Israel to produce Epsom salts (magnesium sulfate) and magnesium chloride.
On land, magnesium salts appear in mineral deposits like dolomite (a magnesium-calcium carbonate) and in evaporite formations left behind by ancient seas. These deposits are mined and refined for both industrial and pharmaceutical use.
Common Types and What They Do
The most widely used magnesium salts each have a distinct profile:
- Magnesium oxide contains the highest percentage of elemental magnesium per gram, but your body absorbs it poorly because it doesn’t dissolve well in water. It’s commonly sold as a supplement and used as an antacid.
- Magnesium sulfate (Epsom salt) dissolves easily in water and is used in hospitals for seizure prevention during pregnancy complications and for correcting dangerously low magnesium levels. At home, it’s a familiar bath soak.
- Magnesium citrate dissolves readily and is one of the better-absorbed oral forms. It’s frequently used as an over-the-counter laxative and as a supplement.
- Magnesium chloride is highly soluble and shows moderate absorption. It appears in supplements, topical products, and as a road de-icer.
- Magnesium hydroxide is the active ingredient in milk of magnesia, used as both an antacid and a laxative.
- Magnesium carbonate neutralizes stomach acid and is used in antacid tablets.
- Magnesium stearate isn’t taken for its magnesium content at all. It serves as a lubricant and anticaking agent in pharmaceutical tablets, capsules, dietary supplements, confectionery, chewing gum, spices, and baking ingredients. It keeps powders from clumping and helps tablets release from molds during manufacturing.
Bioavailability: Which Forms Absorb Best
Not all magnesium salts deliver magnesium to your bloodstream equally. Research consistently shows that organic salts like magnesium citrate are more bioavailable than inorganic salts like magnesium oxide. The tradeoff is that inorganic forms pack more elemental magnesium into each pill, while organic forms dissolve more easily but contain less magnesium per dose.
Lab simulations of human digestion, published in the journal Nutrients, ranked absorption efficiency across several popular supplements. Magnesium glycinate chelate performed best under both fed and fasted conditions. Magnesium chloride and magnesium citrate showed moderate absorption, with citrate performing slightly better when taken with food. Magnesium oxide consistently ranked last for absorption efficiency.
In a human trial from the same study, participants who took a supplement combining magnesium oxide with magnesium glycerophosphate saw a peak blood magnesium increase of 6.2%, compared to 4.6% for magnesium oxide alone. Doubling the combination dose pushed the increase to 8.0%. These are modest differences, but they add up over weeks of daily supplementation, particularly for people trying to correct a deficiency.
Medical Uses
Magnesium salts play several roles in medicine. When taken by mouth, forms like magnesium citrate and magnesium hydroxide draw water into the intestines through osmosis, softening stool and relieving constipation. This osmotic effect is why high doses of almost any magnesium salt can cause loose stools.
In hospitals, magnesium sulfate is used intravenously to prevent seizures in pregnant women with preeclampsia or eclampsia. It’s also given to correct low magnesium levels in the blood and to treat certain heart rhythm abnormalities caused by magnesium deficiency. Every gram of magnesium sulfate delivers about 99 milligrams of elemental magnesium.
Oral magnesium supplements, typically as citrate, glycinate, or oxide, are widely used to maintain adequate magnesium intake. Magnesium is involved in over 300 enzyme reactions in the body, supporting muscle function, nerve signaling, blood sugar regulation, and bone health.
The Epsom Salt Bath Question
Epsom salt baths are one of the most popular home uses for magnesium sulfate, often promoted as a way to absorb magnesium through the skin. The scientific evidence doesn’t support this. A review in the journal Nutrients examined the available research and concluded that transdermal magnesium absorption is “scientifically unsupported.”
The problem is size and biology. A hydrated magnesium ion is roughly 400 times larger than its dehydrated form, making it nearly impossible to pass through skin on its own. Living cells transport magnesium using specialized protein channels, but the dead cells forming the outermost skin layer lack these transporters. Hair follicles and sweat glands could theoretically allow some passage, but they cover only 0.1% to 1% of the skin’s surface. A clinical study testing a magnesium-rich lotion applied three times daily for three days found no change in blood magnesium levels. Epsom salt baths may feel soothing for sore muscles, but the benefit likely comes from the warm water, not from magnesium entering your bloodstream.
Side Effects and Safety
At normal supplemental doses, the most common side effect of magnesium salts is diarrhea, which is a direct result of that same osmotic water-pulling effect used to treat constipation. Nausea can also occur, especially with magnesium oxide on an empty stomach.
True magnesium toxicity is rare in people with healthy kidneys, because the kidneys efficiently filter out excess magnesium. The risk rises sharply in people with impaired kidney function, who can’t clear the mineral fast enough. Early signs of excess magnesium in the blood include flushed skin, nausea, low blood pressure, and general muscle weakness. As levels climb higher, reflexes disappear and muscles become progressively weaker. At very high concentrations, magnesium interferes with the heart’s electrical system, potentially causing dangerous rhythm disturbances. Blood magnesium above 2.2 milliequivalents per liter is considered elevated.
For most people taking oral supplements, reaching toxic levels is extremely unlikely. The intestinal side effects (diarrhea) act as a built-in safety valve, limiting how much magnesium your gut will absorb before flushing the rest out.