That's interesting. Now that I think of it, the nausea feels exactly like what you get from ingesting the tannins in overbrewed black tea. I just assumed my nausea from drinking green tea and from L-Theanine (which I know that green tea contains in a non-negligible quantity). Maybe levels of tannins in green tea are higher than in black tea? In that case the nausea from L-Theanine may be unrelated. Maybe it's a combination of both.