What Is L-Theanine? L-theanine is an amino acid found in tea leaves (Camellia sinensis) and the Bay bolete mushroom (Xerocomus badius).1 L-theanine is also available in supplement form. Clinical trials suggest L-theanine has anti anxiety effects and promotes relaxation, sleep, cognitive function (processing and understanding information), and more.23456 Different types of tea have different amounts […]