If these are supplements, always remember: no regulation. It could be sawdust. Batch to batch, who knows. They should make it a banner above every GNC and Vitamin Shoppe: that bottle could be dried dog shit and you wouldn't know.
To really be sure, you'd need a standard. Sigma Aldritch offers pure theanine at the unhelpful price of a dollar per mg:
http://www.sigmaaldrich.com/catalog/product/sigma/smb00395?lang=en®ion=US which makes you wonder why the store bottle is a thousandth the price.
I tried it too eons ago, and didn't notice anything (yes, I WAS going for some mystical Zen state beyond alpha waves . . . ).
I checked pubmed, and there are actually a LOT of papers on theanine effects (why do people always want to add which isomer? Nobody ever says "I need to check my blood levels of alpha-D-glucose") that I'm too lazy to go through. The first few look like positive anti-stress results.
If you're unfamiliar with pubmed, here's a link:
https://www.ncbi.nlm.nih.gov/pubmed/?term=theanine+anxiety
Most of these results link to journals you need a subscription to, at unbelievable cost, unless you're a student at a university. But usually you just need the abstract.
If you are familiar with it, go ahead and sneer at my condedescension.