I used a few grams of it daily for a few month, it suddenly turned on me, made me anxious and weirdly restlessness one night when combined with a bunch of alcohol. Ever since then its made me feel weird.
It appears to be a low efficacy partial NMDA agonist/weak NMDA antagonist of sorts, it once snapped me out of an unpleasantly intense 2fdck experience. I figured 1.5g of l-theanine might counteract it, and it did, within 20 minutes (sublingually).
In any event high doses of l-theanine does strange things, pro-glutaminergic perhaps. Unpleasant. Makes sense. L-theanine has an umami like taste of glumate (like MSG), massive amounts may act as an NMDA agonist, which is not fun.
Like many naturally occurung compounds, very high doses do the opposite of low doses. My daily giant doses of l-theanine sort of ruined it for me.