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Alli is half-strength Xenical (orlistat), a prescription drug that is taken with meals to prevent the enzyme, lipase, from breaking down fat in the intestines. Undigested fat passes out of the body, and so the calories from fat are not absorbed. Research shows that Xenical has only a modest effect on weight loss and maintenance. The makers of Alli recommend limiting fat to 15 grams at meals to prevent gas with oily spotting, loose stools, and urgent bowel movements. Alli is designed to supplement a low fat diet, regular exercise, and behavior changes.