There are many causes of colic in horses, and sand colic is just one of them. Sand colic is one of the most common causes of horses who have recurrent colic, and it's common in many areas in which horses are kept on sandy soil. Many horses that are fed on the ground and are boarded on sandy soil or that graze on short grass on sandy pastures pick up small amounts of sand as they eat. Over time, this sand accumulates in the colon and causes inflammation, decreased function, and impaction. Several recommended products to remove the sand include psyllium and magnesium sulfate. Psyllium is the ingredient in the human product Metamucil, and magnesium sulfate is commonly known as Epsom salts; both of these products used to remove sand from horse's colon are commonly available over the counter. Of course, the dosage for a horse is much larger in than humans, as routinely a pound of each drug is administered to a horse by a nasogastric tube placed in the horse's stomach.
There is some question as to which of these medications is most effective at removing sand and a study was performed in Helsinki, Finland, to determine the answer. A group of horses diagnosed with sand accumulation were divided into groups and one group was given psyllium, one group was given Epsom salts, and another group was given both. The group that received both psyllium and Epsom salts had the largest decrease in sand in the colon as found on x-rays. So it seems the use of psyllium and Epsom salts together is the most effective treatment for removing sand. However, the large amount of magnesium could cause a problem in horses who have renal disease.