L-theanine is an amino acid that occurs in green and black tea. Some evidence suggests that it is a mild sedative and anti-anxiety agent. A 2016 double-blind study found that people who consumed a beverage containing 200 mg of l-theanine had lower stress response and cortisol levels after undertaking a challenging task than those who received a placebo. It is recommended to start with the lowest effective dosage of l-theanine. Supplements often come in 200 mg capsules. People should not exceed 400 mg without first asking a doctor.