Caffeine and theanine each kept attention up; together, no extra gain
In one trial, 27 people did a two-hour attention task. 50mg of caffeine or 100mg of theanine on its own held errors down against placebo, and the two together did no better than either alone.
What the study found
Caffeine and theanine each cut errors, and together did no better
In 27 people over a two-hour attention task, errors crept upward on placebo. Both 50mg of caffeine and 100mg of L-theanine held those errors down on their own, and taking the two together did no better than either did by itself.
The caveat · Behaviour moved, brain waves did not
A single two-hour task. Theanine's effect showed up in behaviour but not in the brain-wave marker much of this field leans on, so the two kinds of evidence do not line up here. And it disagrees with a similarly dosed trial that did find an advantage for the combination.
What we don't know
What is open is whether pairing theanine with caffeine helps attention generally or only on the brief task that found it. Counting errors over two hours in 27 people, the pair did no better than either alone.