In 1618 a farmer at Epsom in England attempted to give his cows water from a well. This they refused to drink because of the water's bitter taste. However the farmer noticed that the water seemed to heal scratches and rashes. The fame of Epsom salts spread. Eventually they were recognised to be magnesium sulphate, MgSO₄. Black recognized magnesium as an element in 1755. It was isolated by Davy in 1808 who electrolysed a mixture of magnesia (magnesium oxide, MgO) and mercuric oxide (HgO). From the Greek word "Magnesia", a district of Thessaly, where large deposits of magnesium carbonate (MgCO3) were found. Davy's first suggestion for a name was "magnium" but the name "magnesium" is now used.

Magnesium is a very important essential macromineral, even though there are only several ounces in the body (0.05 percent of body weight). It is involved in several hundred enzymatic reactions, many of which contribute to production of energy and cardiovascular function. The great amount of research on magnesium done in the last decade has resulted in major changes in our knowledge. Decreases in magnesium intake have been more prevalent in our American diet with additions of supplemental vitamin D and calcium, dietary phosphorus, and refined or processed carbohydrate foods. Drinking soft water decreases magnesium intake, while diuretic drugs cause magnesium loss, as do alcohol, caffeine, and sugar. Decreased blood and tissue levels of magnesium have been shown to be related to high blood pressure, kidney stones, heart disease and, particularly, heart attacks due to coronary artery spasm (magnesium helps relax and dilate coronary arteries). Studies have indicated that a decreased concentration of magnesium is found in the heart and blood of heart attack victims, though it is not clear whether this is a cause or a result of the problem. Magnesium's role in alleviating premenstrual syndrome (PMS) has made big news as well.

Drinking soft water decreases magnesium intake, while diuretic drugs cause magnesium loss, as do alcohol, caffeine, and sugar. Decreased blood and tissue levels of magnesium have been shown to be related to high blood pressure, kidney stones, heart disease and, particularly, heart attacks due to coronary artery spasm (magnesium helps relax and dilate coronary arteries).

Magnesium helps with formation of bone and teeth and assists the absorption of calcium and potassium. Where calcium stimulates the muscles, magnesium is used to relax the muscles. It is further needed for cellular metabolism and the production of energy through its help with enzyme activity. It is used for muscle tone of the heart and assists in controlling blood pressure. Together with vitamin B12, it may prevent calcium oxalate kidney stones. It helps prevent depression, dizziness, muscle twitching, and pre-menstrual syndrome. It can help prevent the calcification of soft tissue and may help prevent cardiovascular disease, osteoporosis, and certain forms of cancer, and it may reduce cholesterol levels. Magnesium assists the parathyroid gland to process vitamin D, and a shortage here can cause absorption problems with calcium.

A severe deficiency caused by mal-absorption, chronic alcoholism, renal dysfunction, or the use of certain medications can cause neuromuscular manifestations, and personality changes can occur. Many cardiovascular problems are indicated with magnesium in short supply and rapid heartbeats as well as fatigue, irritability, and seizure can occur. Insomnia, poor memory, painful periods, depression, hypertension and confusion may also be indicative shortage of magnesium in the body. It is used for the management of premature labor, and for the prophylaxis and treatment of seizures in toxemia of pregnancy. A deficiency may also be a contributing factor to incontinence in older people and bedwetting in children. Bruxism can predispose one to cardiac-rhythm abnormalities, confusion, convulsions, diarrhea, hallucinations, irritability, loss of appetite, nausea, nervousness. Over time a deficiency may create cardiovascular problems, skin disorders, tachycardia, tremors, uncontrolled muscle contractions, vomiting. Deficient in alcoholics. Average American gets only 3/4 of RDA.

Many factors affect magnesium availability from foods. One is the amount of magnesium in the soil in which the food is grown. Much magnesium can be lost in the processing and refining of foods and in making oils from the magnesium-rich nuts and seeds. Nearly 85 percent of the magnesium in grains is lost during the milling of flours. Soaking and boiling foods can leach magnesium into the water, so the "pot liquor" from cooking vegetables may be high in magnesium and other minerals. Oxalic acid in vegetables such as spinach and chard and phytic acid in some grains may form insoluble salts with magnesium, causing it to be eliminated rather than absorbed. For these reasons and those previously discussed, many people get insufficient magnesium from their diets. Magnesium is found in dairy products, fish, meat and seafood, as well as in legumes, apples, apricots, avocados, bananas, whole grain cereals, nuts, dark green vegetables, and cocoa, while hard water and mineral water may also supply it in fair quantities.