Overview

You may have heard commercials and read ads about diet pills that claim to literally melt away fat. But the only diet drug that effectively traps fat from being digested is orlistat, according to the National Institute of Diabetes and Digestive and Kidney Diseases. Orlistat, like any other medication, does carry the risk of side effects, so investigate the implications carefully and talk to your doctor before taking any brand of orlistat. No version of orlistat is suitable for pregnant women, nursing women, people under the age of 18 or people with compromised immune systems or liver problems.

Alli Basics

Alli is an over-the-counter brand of orlistat. Each pill consists of 60 mg of orlistat and should be taken before you eat any meals containing fat. Once you take the pill, the ingredients travel to your digestive system and block fat from being digested; you pass the undigested fat out of your body through bowel movements. If you take Alli, you can expect to lose 3 to 5 lbs more per year than you would have just relying on diet and exercise, according to the Mayo Clinic.

Xenical Basics

Xenical consists of 120 mg of orlistat per pill and requires a doctor's prescription. If you take Xenical, you can expect to lose 5 to 7 lbs more per year than you would have with just good diet and exercise habits, notes the Mayo Clinic. But you should not take Xenical before eating meals with more than 15 grams of fat and should skip your dose if you are eating a non-fat or very low-fat meal. All forms of orlistat are approved for use of up to two years.

Potential Side Effects

Orlistat, like any other drug, can cause allergic reactions or unpleasant side effects. Common side effects include headache, dry mouth and an unpleasant taste in the mouth, according to PubMed Health. But the most common patient complaints involve problems with bowel movements; problems such as excessive bowel movements, urgent bowel movements, oily stools, fatty stools, and gas that causes oily spotting onto your underwear can be minimized or eliminate if you eat low-fat meals while taking orlistat.

Liver Injury Potential

In rare cases, taking orlistat can cause serious liver injury, warns the National Institute on Diabetes and Digestive and Kidney Diseases. In 2009, representatives of the Food and Drug Administration began investigating patient claims that orlistat damaged their livers. As of 2011, the drug was still on the market. If you experience jaundice, light-colored stools or dark-colored urine after taking orlistat, seek immediate medical assistance.