Metamucil is a bulk fiber laxative for treating occasional constipation and restoring regularity. Its active ingredient is psyllium husk, a natural plant fiber with a high percentage of soluble fiber. Available over the counter (OTC), Metamucil is the # 1 Doctor Recommended Fiber Supplement. The 100 percent natural psyllium fiber in Metamucil works with your body to gently soften stool and maintain regularity. The fiber absorbs water and softens and adds bulk to stools for easier elimination. Regularity can be restored within 12 to 72 hours.