Caffeine and L-theanine
Caffeine plus L-theanine has a real, small human study on attention and mood. The combination beat placebo on some tasks and was not the same as caffeine alone. It will not replace sleep. People who are pregnant, anxious, or sensitive to caffeine should not treat a stack as harmless because theanine is in tea. The study dose is in the paper, and a random focus gummy may not match it.
Tea already contains both caffeine and L-theanine. The supplement version is L-theanine plus caffeine in capsule form, at doses higher than a cup of tea.
The trial
Haskell and colleagues tested L-theanine, caffeine, and the combination against placebo on cognition and mood (PMID 18006208). The combination had effects that neither component fully copied. That is the evidence. It is one controlled study of healthy adults, not a semester-long focus program and not a treatment for ADHD.
Practical limits
Caffeine later in the day costs sleep, and lost sleep costs the next day's attention. Theanine does not cancel that. If a product hides the caffeine milligrams inside a proprietary blend, skip it. Match a published dose or do not bother. Blood pressure, pregnancy, and panic symptoms are reasons to leave caffeine stacks alone.
Sources
- Haskell CF, et al. The effects of L-theanine, caffeine and their combination on cognition and mood. Biol Psychol. 2008. PMID: 18006208.