I tried taking L-Theanine for about 5 days.  30 minutes after taking it each day, I would get very nauseous and usually throw up.  I tried varying the dosage from 200mg down to 100mg and then 75mg, I would still get the nausea, no matter if I had an empty stomach or not.  I also get the same symptom when drinking more than a cup or so of green tea, and has for years (though I do remember being able to stomach the stuff in the past).  It would be interesting to hear if this happens to anyone else.