The European Food Safety Authority found there was insufficient evidence for a causal relationship between theanine consumption and improved cognitive function, alleviation of psychological stress, maintenance of normal sleep, or reduction of menstrual discomfort.

[citation needed] Relative to theanine, the opposite (D-, R-) enantiomer is largely absent from the literature,[2] except implicitly.

While natural extracts that are not harshly treated are presumed to contain only the biosynthetic L- enantiomeric form, mishandled isolates and racemic chemical preparations of theanines necessarily contain both theanine and its D-enantiomer (and from racemic syntheses, in equal proportion), and studies have suggested that the D-isomer may actually predominate in some commercial supplement preparations.

[9][10] Amino acid racemization in aqueous media is a well-established chemical process promoted by elevated temperature and non-neutral pH values; prolonged heating of Camellia extracts—possible for oversteeped teas and in undisclosed commercial preparative processes—has been reported to result in increasing racemization of theanine to give increasing proportions of the nonnatural D-theanine, up to equal proportions of each enantiomer.

It was discovered as a constituent of tea (Camellia sinensis) in 1949, and in 1950 a laboratory in Kyoto[11] successfully isolated it from gyokuro leaf, which has high theanine content.

[11][23] In the United States, the FDA considers it to be GRAS and allows its use as an ingredient in dietary supplements or foods up to a maximum of 250 mg per serving.

In 2003, the German Federal Institute for Risk Assessment (Bundesinstitut für Risikobewertung, BfR) objected to the addition of isolated theanine to beverages.

[28] Evidence from a systematic review of randomised controlled trials suggests that L-theanine supplementation may be more effective than placebo in alleviating psychopathological symptoms associated with anxiety disorders, ADHD, and schizophrenia.