L-Theanine to Support Relaxation and Mood for Cancer Patients in Surveillance

Inclusion Criteria: * Documented informed consent of the participant and/or legally authorized representative. * Assent, when appropriate, will be obtained per institutional guidelines * Willingness to comply with all study interventions including the use of L-theanine and follow-up assessments * Age: ≥ 18 years * Ability to read and understand English or Spanish for questionnaires * Patients must be a cancer patient who has completed treatment and has been in surveillance for at least 6 months * Participants must not have used any herbs or supplements in the past 30 days * Participants must report an anxiety score of \> 3 on Visual Analog Scale - Anxiety * Participants must not have had any changes to their psychiatric medications within the past 60 days * Women of childbearing potential (WOCBP): negative urine or serum pregnancy test. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required * Agreement by females and males of childbearing potential to use an effective method of birth control or abstain from heterosexual activity for the course of the study through at least 1 month after the last dose of protocol therapy. * Childbearing potential defined as not being surgically sterilized (men and women) or have not been free from menses for \> 1 year (women only) Exclusion Criteria: * Concurrent use of other alternative medicines, including herbal agents and high-dose vitamins and minerals, unless taken to correct a documented vitamin or mineral insufficiency * Chemotherapy, radiation therapy, biological therapy, immunotherapy, or any other systemic treatment excluding hormonal therapy (must be on a stable dose of hormonal therapy for at least 60 days) * Any patients taking bortezomib, as L-theanine can decrease effectiveness * Any patients currently enrolled in other clinical trials that might interfere with the results of this study * History of allergic reactions attributed to compounds of similar chemical or biologic composition to study agent * Any patients with an ongoing active/unstable psychiatric condition, depressive/bi-polar related disorders, anxiety, psychosis disorders, or substance use that may interfere with the ability to participate in the study as outlined in the study procedures section of the protocol * Any patients with chronically unstable blood pressure or chronic low blood pressure * Diagnosis of Gilbert's disease * Females only: Pregnant or breastfeeding * Any other condition that would, in the Investigator's judgment, contraindicate the patient's participation in the clinical study due to safety concerns with clinical study procedures * Prospective participants who, in the opinion of the investigator, may not be able to comply with all study procedures (including compliance issues related to feasibility/logistics)