Ephedrine, caffeine, and aspirin are often combined by dieters or bodybuilders to stimulate thermongenesis, the biological process which burns calories. Herbal equivalents of the ECA stack have been made from Ma Huang, guarana, and willow's bark. The caffeine and aspirin significantly increase the thermogenic properties of ephedra/ephedrine, but also introduce greater risk of problems. Every authoritatave source imaginable recommends extreme caution when using the ECA stack, and warns against using it for too long. Exact ratios of the three ingredients to each other and the efficacy of the herbal substitutes, especially willow's bark, are among the topics endlessly debated by proponents and critics of the ECA stack.