[binary132]

Anecdotally, my theanine use correlated strongly with my development of terrible chronic migraines which lasted years. But I was also experiencing severe stress during that time, so it could just be that. However, when I stopped taking L-theanine, my headaches did lessen in severity and frequency, so there may be something there and I’ve heard others mention headache in relation to it.

[smokeydoe]

This is also my experience. I would get pretty bad next day headaches especially when taken at night.

[sheefrex]

Yes, apparently headaches are listed as one of the possible side-effects