Orlistat is a pharmaceutical drug, sold over-the-counter under the name Alli, and under prescription as Xenical.
How it works: This weight loss pill works by inhibiting the breakdown of fat in the gut, making you take in fewer calories from fat.
Effectiveness: According to a big review of 11 studies, orlistat can increase weight loss by 6 pounds (2.7 kg) compared to a dummy pill
Other benefits: Orlistat has been shown to reduce blood pressure slightly, and reduced the risk of developing type 2 diabetes by 37% in one study
Side effects: This drug has many digestive side effects, including loose, oily stools, flatulence, frequent bowel movements that are hard to control, and others. It may also contribute to deficiency in fat-soluble vitamins, such as vitamins A, D, E and K.
It is usually recommended to follow a low-fat diet while taking orlistat, in order to minimize side effects.
Interestingly, a low carb diet (without drugs) has been shown to be as effective as both orlistat and a low-fat diet combined (16).