The notion of a "fat blocker" pill may sound like a boon to those who are trying to lose weight. The idea that a simple pill could keep your body from absorbing the calories in the food that you eat sounds suspiciously like a magic pill that makes you skinny while you pig out. There are two medications that are technically fat blockers, and while they have shown some promise as weight loss aids, they are anything but magic.
Orlistat was the first fat-blocking drug approved by the U.S. Food and Drug Administration and is available by prescription, and a half-strength version is also approved for over-the-counter sale. Orlistat works by disabling the digestive enzyme that breaks fat down into molecules small enough to be absorbed into the bloodstream so the fat passes through the digestive system and is excreted with the stool. The drug does not block every drop of fat, so you must still follow a low-fat diet while taking the pill.
Chitosan is a supplement, and as such, is not regulated by the FDA. It is extracted from the shells of crustaceans and chemically treated to make it attract and absorb fats. It was formerly used to purify water because it would soak up the foreign substances and form a film on the surface that could be easily removed. It works basically the same way when taken internally -- the fat you eat is attracted to the chitosan, which soaks it up and holds it until it is excreted. This prevents the body from absorbing the fat.
To be approved by the FDA, drug manufacturers must demonstrate that their product works better than a placebo -- so in those terms, orlistat works. It is not the magic pill many have hoped for, however. It doesn't work unless a low-calorie, low-fat diet plan and regular exercise are followed at the same time, and according to Dr. Donald Hensrud of the Mayo Clinic, you can only expect to lose about 5 to 7 more pounds over the course of a year than someone who diets and exercises without taking orlistat. The jury is still out on chitosan -- there is anecdotal evidence of it's effectiveness, but it has not been backed up by a high-quality scientific study.
Chitosan is thought to be generally safe and without recognized side effects, but because it is unregulated, consult your doctor before use. Orlistat is notorious for causing anal leakage, oily spotting and urgent stools, especially after meals containing more than 15 g of fat. It can also cause abdominal pain, headaches and changes in bowel habits. Sometimes these symptoms go away after continued use of the drug, but other people experience them throughout the course of treatment.