Alli is a weight loss medication that is available over the counter is some countries.  The same medication (orlistat) is also available as the prescription medication, Xenical.  Orlistat is derived from the bacterium Streptomyces toxytricini.  The mechanism of action is for the active ingredient to inhibit the action of pancreatic lipases.  Pancreatic lipases are chemicals that are released into your small intestine to digest fats.
When the orlistat binds to the lipases, they no longer attach to fat molecules.  The end result is that the fat is not absorbed and is expelled during normal bowel movements.

Orlistat is intended for use in conjunction with a program of healthy eating and exercise that is supervised by a qualified health care practitioner.  This does not change just because Alli is available over the counter.  It is not suitable for everyone – it is intended to be used on obese people (not if BMI below thirty) who are not pregnant or breastfeeding.

Additionally, if you have an organic cause to your obesity (eg hypothyroidism), if you are taking certain prescription medications or if you have any malabsorption syndromes or impaired bile flow then it may be dangerous for you to use this supplement.

As this supplement is designed to prevent fat absorption, it is recommended that you take a multivitamin that contains the fat soluble vitamins to correct for any reduced intake associated with decreased fat absorption.

The internet contains some personal stories regarding the side effects of Alli.  The double-blind, randomized, control studies that are required by law before these products can be sold report a very low rate of adverse reactions.  Approximately 5% of people using Alli will experience gastrointestinal symptoms.  This ranges from slightly smellier faeces that float to some level of spotting of an oily residue.  This low rate of adverse reaction occurs when the product is used as directed.

Directions for using Alli include the dosage, that people with malabsorption issues shouldn’t use it and that it is part of a healthy eating and exercise program.  A fat intake of 30% of your calories should not be exceeded while using orlistat.  If you exceed this then a larger amount of fat will pass undigested through your intestines and you are more likely to experience an adverse reaction.

Using a fat binder as directed, and as part of a program of healthy eating and exercise will improve your weight loss efforts and help you maintain the weight you achieve.  Orlistat has been shown to be effective in maintaining weight goals for four years in scientific research studies.  Used appropriately, it may help you achieve your goals.