Safety

Based on the results of the clinical studies, L-theanine is most effective in the range of 50-200 mg, with the effect being felt within 30 minutes and lasting for 8-10 hours. Individuals with high stress levels may increase their dosage of L-theanine to at least 100 mg, with no more than 600 mg being taken in a six hour period. FDA recommends a maximum dose of 1200 mg daily, although the reason for this limit is not clear, due to its demonstrated safety. There are no known adverse reactions to L-theanine and no drug interactions have been reported. L-theanine is not affected by food and may be taken anytime, as needed. Because it has a mild taste, capsules may be opened and dissolved in water. Although it is probably safe for pregnant women and nursing mothers, we discourage its use by them pending conclusive research.
 
No adverse side effects are associated with theanine consumption (3-4 cups of green tea are expected to contain 100-200 mg of theanine) - making it one of the leading natural choices for promoting relaxation without the sedating effects of depressant drugs and herbs. When considering the potential benefits of theanine as an anti-stress or anti-cortisol supplement, it is important to distinguish its non-sedating relaxation benefits from the tranquilizing effects of other "relaxing" supplements such as valerian and kava, which are actually mild central central-nervous nervous-system depressants.
 
L–Theanine has no cautions or drug interactions with the exception of certain chemotherapy agents whereby it enhances the chemotherapy effects.  If you are receiving chemotherapy, check with your doctor.