The main component, Ephedrine, is the main active ingredient for weight loss and the caffeine augments its efficacy. The aspirin is added to prevent blood clotting (which may be a side effect of ephedrine) and to aid in the signal transduction of ephedrine via prostaglandin inhibition.
Some proponents of the ECA stack recommend replacing the Aspirin in the stack with either Fish Oil or White willow bark, as Aspirin is linked to various organ side effects and the previous two compounds share the same anti-clotting mechanisms.
Studies have shown that the combination of Ephedrine and Caffeine is superior for fat loss than taking each individually. As ephedrine can increase heart rate and caffeine may increase blood pressure (in a naive user), the combination tends to have an additive rather than synergistic toxicological profile.