Stacker 2 and 3 weight-loss supplements are based on the original ECA stack -- a combination of ephedrine, caffeine and aspirin.

Since the FDA banned ephedrine (the "E"), these supplements have been reformulated and bitter orange and thyroid-stimulants called Guggulsterones have been added to take the place of ephedrine.

The difference between Stacker 2 and Stacker 3 is that Stacker 3 also contains chitosan, a type of "fat blocker" which Stacker 2 does not contain.

Chitosan is a fiber extracted from the shells of shellfish such as shrimp and crabs, which have been ground into a pulverous powder.

The claim is that chitosan is able to "soak up" or absorb anywhere from six to ten times its weight in fat and oils, binding with the fat molecules and converting them into a form the human body does not absorb. This fat is then excreted in the stool.

However, we are not aware of any studies showing that chitosan works to promote weight loss. And in a small study reported in 2003, Dr. Judith Stern, co-founder of the American Obesity Association, reported "the effect of chitosan on fat absorption is clinically negligible.The fat trapping claims associated with chitosan are unsubstantiated."

So that pretty much leaves both Stacker 2 and Stacker 3 to rely on the stimulants substituted for ephedrine. It is unclear whether bitter orange acts similarly to ephedrine in raising heart rate, respiratory rate and blood pressure, but if it does, either of the Stackers could be risky for people with arrhythmias and high blood pressure.