Metamucil typically starts working 12 to 72 hours after taking it, states Drugs.com. The laxative contains psyllium, which absorbs liquid within the intestines. This creates soft, bulky stool that passes through the system more comfortably. Sometimes physicians recommend Metamucil to help treat high cholesterol and irritable bowel syndrome, notes WebMD.
As a treatment for occasional constipation, an individual should take 1 rounded teaspoon of powdered Metamucil mixed with 8 ounces of water no more than three times a day, according to Procter & Gamble. If the amount of liquid is not sufficient, psyllium swells in the throat, warns Drugs.com. This is a choking hazard.