Evaluation of Safety of Long-term and Excessive Intake of L-Cystine and L-Theanine in Healthy Adult Subjects

To evaluate the safety of L-cystine and L-theanine in humans, we carried out a clinical trial of high-dose and long-term administration in the form of a controlled randomized double-blind study. Forty healthy subjects, 20 males and 20 females, were given three sachets of food containing L-cystine and L-theanine or three sachets of placebo once a day for four weeks. The dosage of L-cystine and L-theanine was three times the dosage previously shown to exert significant enhancing effect on the immune response to influenza vaccine inoculation. High-dose administration was found to cause no significant clinical changes, no abnormal findings in clinical laboratory data, and no adverse effects. Although some statistically significant changes were observed in laboratory data, all were clinically insignificant variations within the normal range. It was therefore concluded that L-cystine and L-theanine are safe when administered for four weeks at daily doses of up to 2,100mg of L-cystine and 840mg of L-theanine.