Psyllium is a bulk-forming laxative and is high in both fibre and mucilage. Psyllium seeds contain 10–30% mucilage. The laxative properties of psyllium are due to the swelling of the husk when it comes in contact with water. This forms a gelatinous mass that keeps faeces hydrated and soft, provided it is taken with sufficient water. The resulting bulk stimulates a reflex contraction of the walls of the bowel, followed by emptying.¹

Psyllium is a common ingredient in over-the-counter bulk laxative products. One preliminary trial found that psyllium seeds relieved constipation when it was due to lifestyle factors (e.g., inadequate fibre, sedentary lifestyle), but not when an actual disease was the cause.² Numerous double-blind trials have found that supplementation with psyllium can lower total cholesterol and LDL (“bad”) cholesterol.³ However, levels of HDL (“good”) cholesterol are not affected by psyllium supplementation.⁴ The cholesterol-lowering effect of psyllium has been reported in children,⁵ as well as in adults.⁶ Psyllium supplementation has also improved blood sugar levels in some people with diabetes.^{7, 8, 9} The soluble fibre component of psyllium is believed to account for this effect.

In a double-blind trial, people with ulcerative colitis had a reduction in symptoms such as bleeding and remained in remission longer when they took 20 grams of ground psyllium seeds twice daily with water compared to the use of the medication mesalamine alone.¹⁰ Also, the combination of the two was slightly more effective than either alone.