Two opposing actions that complement each other
Caffeine is a stimulant that works mainly by blocking adenosine receptors, the receptors that normally promote drowsiness, which raises alertness but can also produce jitter, a faster heart rate, and anxiety in sensitive people. L-theanine is an amino acid from tea that promotes alpha-wave brain activity associated with relaxed focus and is generally described as calming without being sedating. Pairing them is built on the idea that theanine offsets caffeine's rough edges.
What the combination is reported to do
Controlled studies, several using a roughly 1 to 2 ratio of caffeine to theanine (for example 50 mg caffeine with 100 mg theanine), report better performance on attention-demanding tasks and reduced subjective jitter compared with caffeine alone. The common summary is that theanine smooths the caffeine peak, sustaining alertness while dampening the overstimulated feeling.
Individual responses vary, and caffeine itself affects sleep, blood pressure, and anxiety. This is general educational information, not medical advice. People with heart conditions, anxiety disorders, or who are pregnant should consult a qualified professional about caffeine intake.