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Magnesium sulfate as a medication is used to treat and prevent low blood magnesium and seizures in women with eclampsia. It is also used in the treatment of torsades de pointes, severe asthma exacerbations, constipation, and barium poisoning. It is given by injection into a vein or muscle as well as by mouth. As epsom salts, it is also used for mineral baths.

Common side effects include low blood pressure, skin flushing, and low blood calcium. Other side effects may include vomiting, muscle weakness, and decreased breathing. While there is evidence that use during pregnancy may harm the baby, the benefits in certain conditions are greater than the risks. Its use during breastfeeding is deemed to be safe. Magnesium sulfate for medical use is the magnesium sulfate heptahydrate salt. The way it works is not fully understood, but is believed to involve depressing the action of neurons.

Magnesium sulfate is a common mineral pharmaceutical preparation of magnesium, commonly known as Epsom salt, used both externally and internally. Magnesium sulfate is highly water-soluble and solubility is inhibited with lipids typically used in lotions. Lotions often employ the use of emulsions or suspensions to include both oil and water-soluble ingredients. Hence, magnesium sulfate in a lotion may not be as freely available to migrate to the skin nor to be absorbed through the skin, hence both studies may properly suggest absorption or lack thereof as a function of the carrier (in a water solution vs. in an oil emulsion/suspension). Temperature and concentration gradients may also be contributing factors to absorption.

Epsom salt is used as bath salts and for isolation tanks. Magnesium sulfate is the main preparation of intravenous magnesium.

Internal uses include:

1. Oral magnesium sulfate is commonly used as a saline laxative or osmotic purgative.
2. Replacement therapy for hypomagnesemia
3. Magnesium sulfate is a antiarrhythmic agent for torsades de pointes in cardiac arrest under the ECC guidelines and for managing quinidine-induced arrhythmias.
4. As a bronchodilator after beta-agonist and anticholinergic agents have been tried, e.g. in severe exacerbations of asthma, magnesium sulfate can be nebulized to reduce the symptoms of acute asthma. It is commonly administered via the intravenous route for the management of severe asthma attacks.
5. Magnesium sulfate is effective in decreasing the risk that pre-eclampsia progresses to eclampsia. IV magnesium sulfate is used to prevent and treat seizures of eclampsia. It reduces the systolic blood pressure but doesn't alter the diastolic blood pressure, so the blood perfusion to the fetus isn't compromised. It is also commonly used for eclampsia where compared to diazepam or phenytoin it results in better outcomes.

An overdose of magnesium causes hypermagnesemia.