Psyllium husk refers to the outer portion of the seeds of Plantago ovata, a plant in the plantain family. The husk is extremely hygroscopic, meaning it absorbs many times its weight in water. As a supplement, psyllium is a source of soluble fiber that improves transit time in the gut.

As such, psyllium is a featured ingredient in over-the-counter laxatives and fiber supplements such as Metamucil.

Powdered psyllium husk is also used as a thickening agent in foods. The powder is also used as a binding agent in gluten-free baking mixes.