Latin name: Ilex guayusa. Non-jittery stimulating leaf with caffeine, theanine, theobromine, and loads of antioxidants. Guayusa (pronounced why-YOU-sah) is a tree leaf that indigenous people in the Ecuadorian Amazon have used for thousands of years for the clear, focused energy that it provides. In addition to caffeine, Guayusa contains theobromine (also found in dark chocolate) and antioxidants (substances that can neutralise damaging free radicals in the body.).