Ephedra's active ingredient, the alkaloid ephedrine, is similar to the most popular alkaloid of all — caffeine. A central nervous system stimulant that constricts blood vessels, caffeine is contained in the leaves or seeds of six herbs better known as beverages than as plants: coffee, tea, kola, cocoa, maté and guarana. And here's a tip if your morning cup isn't doing the trick — preparation, more than plant source, determines caffeine content. Tea leaves contain twice the caffeine of coffee beans, yet a cup of tea delivers only half the kick. That's because slowly boiling cold water and ground coffee (a decoction) extracts more caffeine than adding boiling water to tea leaves (an infusion).