Epsom salts, also known as magnesium sulphate, can be bought fairly inexpensively at drug stores. They are used in humans mostly as laxatives and an anti-inflammatory agent, but in horses, they are mostly used to help feet. They are also used as a supplement for horses, but this has to be done very carefully, because too much magnesium sulphate can be detrimental.
Epsom salts are natural minerals. They help the body's muscles and nerves by reducing lactic acid and acting as a calming agent. Magnesium sulphate also increases the amount of liquid in the intestines, which helps bowel movements. Horses can be magnesium-sulphate deficient; those with this deficiency will act uneasy, nervous and have twitching muscles.
Horses are increasingly being fed epsom salts as a supplement to help nerves and an issue called "tying up." Tying up is when a horse's muscles fill up with so much lactic acid that it makes it hard--and, sometimes, impossible--for it to move. Studies to determine more about this phenomenon are ongoing, but feeding epsom salts seems to help relieve the condition. Horses are also being given epsom salts to decrease obesity, which is helping with a lot of horses who would typically founder.
While magnesium sulphate has many benefits when fed as a supplement, it also has its bad points. It is primarily used as a laxative, and if too much is fed to a horse, it will give the horse diarrhoea. Excess magnesium is excreted in the urine but can lead to diarrhoea and possibly heart and renal issues.
Epsom salts are also great as an anti-inflammatory and drying agent when used externally. For a horse with a suspected abscess in their hoof, soak the foot in a mixture of warm water and Epsom salt. The Epsom salt relieves some the pain and pressure of the abscess and dries out the hoof to draw the abscess out.
Epsom salts can also be put into a hoof-packing-type poultice. This serves the same purpose as soaking the hoof in Epsom salt and water, but lasts as long as the poultice boot will stay on. Generally, you mix hoof poultice and Epsom salts together, pack the hoof with this substance, and then use a feed sack and duct tape to boot the foot.