Alli is a new over-the-counter diet pill ($49.95+ per bottle) that works by preventing some of the fat you ingest from being absorbed. What's the catch? You might poop your pants (seriously). Hilarious but authentic quotes from the official Alli site:

"Until you have a sense of any treatment effects, it's probably a smart idea to wear dark pants, and bring a change of clothes with you to work"

"You may not usually get gassy, but it's a possibility when you take alli. The bathroom is really the best place to go when that happens".

"The excess fat that passes out of your body is not harmful. In fact, you may recognize it as something that looks like the oil on top of a pizza." Source

Ingredients & How it Works

The active ingredient in Alli is Orlistat. Orlistat works by blocking the absorption of fat. Orlistat is also found in the prescription weight loss drug Xenical, which has a stronger dose than Alli.

GlaxoSmithKline (GSK) emphasizes that you need to exercise and eat a healthy diet to lose weight while taking Alli. Supposedly you can lose 50% more weight when taking Alli while exercising and eating right, when compared to exercise & diet changes alone.

I'd be interested to find out if this 50% increase that Alli claims is superior to the effect of fiber-supplements such as Glucomannan, or even caffeine/green tea extract. I'll dig up some studies on competing products that also require "exercise and diet modifications" and post a comparison soon.

Important

The FDA recommends taking a multivitamin with Alli, since the active ingredient can reduce absorption of certain nutrients. They also warn: "OTC Orlistat is not for people who have problems absorbing food or for those who are not overweight."

From the Alli official site - Includes the soon-to-be notorious "bring a change of pants" comments.

Forum discussion from coolrunning.com about Alli - Particularly the unsavory side-effects.

FDA Press Release on Alli - Basic info on Alli