Orlistat (Xenical, Alli) works by slowing down the production of an enzyme (lipase) that breaks down fat in the stomach. This reduces the amount of dietary fat that can be digested by about 30 percent. Orlistat only works within the intestines, and not all over the body like the other diet drugs. Side effects of Orlistat are more common in those who do not follow a low-fat diet.

Diarrhea and cramps are common side effects in those who eat high fat foods when taking Orlistat. Other reported effects include oily spotting, flatulence with discharge, fecal urgency, fatty or oily stool, oily evacuation, increased defecation, and fecal incontinence. It may also interfere with absorption of vitamins A, D, and E.

Most experts recommend additional vitamin supplements for those taking this drug. Orlistat appears to be safe and effective for long-term weight loss.

At the end of the first year, many orlistat users can see an average 10% drop in body weight. Weight loss induced by orlistat can significantly reduce elevated blood pressure in overweight people. Weight loss with orlistat can also produce a significantly greater reduction in cholesterol levels.