Psyllium husks consist of hemicellulose, which is insoluble, meaning that it is not broken down and absorbed during digestion. When this husk comes into contact with moisture, it swells into a gelatinous mass. Upon swelling, it is thought to bind sand (agglutination) in the equine gastrointestinal tract and move it out with the manure.

The efficacy of psyllium husks in the removal of sand from the equine gut remains in question, as studies of its use have yielded mixed results. Nevertheless, many vets including myself recommend the periodic feeding of a psyllium supplement in certain cases to prevent or lessen the severity of sand colic. Regardless, it is very important to try to prevent the ingestion of sand in the first place.

Psyllium is also sometimes given in large doses by a vet though a stomach tube in attempt to move large quantities of sand out of a horse diagnosed with sand colic. Following treatment, a higher amount of sand should be evident in the horse's manure, and their clinical signs should improve (less pain, lower heart rate).