What is the effect of combining L-theanine with caffeine?

As described above, theanine generally has calming or relaxing effects.^[1][2][3][4] On the other hand, caffeine is a powerful central nervous system stimulant that is added to some energy drinks and naturally found in tea, coffee, and chocolate. Because tea contains both L-theanine and caffeine, some clinical studies have investigated the effects of theanine and caffeine individually and in combination. The current evidence shows that theanine might diminish some of the less desirable physiological effects of caffeine. For example, L-theanine has been shown to blunt both caffeine’s blood-pressure-increasing effects^[5] and its sleep-disrupting effects.^[6] However, theanine does not appear to blunt the beneficial effects of caffeine on cognitive performance. Instead, the above-described beneficial effects of theanine on some aspects of cognitive function,^[7][8][3][4] such as attention, have been found to be enhanced when caffeine is simultaneously ingested.^{[9][10][11][12][13]} That said, not all studies have found this synergistic effect.^[14][5] Overall, dose-response studies of L-theanine and caffeine are currently lacking, and the existing studies are small; larger randomized controlled trials are needed to clarify the interaction between theanine and caffeine. Furthermore, the influence of theanine on the exercise-related performance-enhancing effect of caffeine is currently unclear.