Not highly effective. Works as a fat blocker. Can help at losing 5-8% of excess body weight if used long term and if can remember to take before a fatty meal. Can have some unpleasant side affects such as oily foul smelling stool, increased flatulence and sometimes incontinence when passing gas. Probably better for short term use. No studies show long term (greater than 5 yr) effective weight loss.
Effective somewhat. Xenical is a drug that promotes loss of weight by preventing the digestion and absorption of fat in food. In the intestine, an enzyme called lipase (produced primarily by the pancreas) breaks apart fat in food so that it can be absorbed into the body. Orlistat blocks the action of lipase and thereby prevents the breakup and absorption of fat. Orlistat blocks absorption of about 25% fat in food.