Nootropics
L-theanine
L-theanine and caffeine.
L-theanine appears to be both very safe and effective. It has a long history of use, an LD50 estimated to be over 1000 times larger than a standard therapeutic dose and no known contraindications with any food or drug. It may well be health-promoting. ref ref ref
The science behind the L-theanine-caffeine combination seems almost equally compelling. L-theanine seems to mitigate the negative aspects of caffeine, such as anxiety, increased blood pressure and diminished sleep quality, while improving upon the positive aspects. Its ability to enhance attention, over and above that of caffeine alone, has been repeatedly verified.
For beginners without a caffeine tolerance it's recommended to start with 50mg caffeine and 100mg of L-theanine. Once in the morning and once around noon.