L-Theanine

L-theanine is a non-protein amino acid found in green and black tea (Camellia sinensis). It is sold as a stand-alone supplement and as an ingredient in "focus" and sleep blends.

Most of the research falls into three groups. The largest looks at the combination with caffeine, where a meta-analysis of randomised trials found moderate effect sizes for alertness and attention switching accuracy in the first two hours [2]. Not every outcome improves: one trial found better task switching but no change in intersensory attention or subjective alertness [4]. A 2025 meta-analysis concluded that theanine plus caffeine, and theanine alone, could benefit cognition and mood [5]. A second group looks at stress and anxiety. A systematic review of nine randomised trials concluded that 200 to 400 mg a day may reduce stress and anxiety in people under stress [1]. In a four-week crossover trial in healthy adults, 200 mg a day lowered depression, anxiety and sleep-quality scores and improved verbal fluency [6]. The third and newest looks at sleep, where a 2025 meta-analysis of 18 trials found small improvements in subjective sleep onset, daytime dysfunction and overall sleep quality [3].

Its mechanism in humans is not settled. L-theanine is structurally related to glutamate. Reviews propose effects on glutamate signalling and on GABA, serotonin and dopamine levels as the basis of its anxiolytic and anti-stress effects, but most of that evidence comes from animals [7].

Why it pairs well with caffeine is also unresolved. In one EEG study, theanine and caffeine each enlarged an attention-related brain response, and together they enlarged it further. The study found no statistical interaction between them, which suggests their effects simply add together rather than one amplifying the other [8].

L-theanine has a clean safety record in the trial literature. Daily doses of 200 to 400 mg for up to eight weeks are described as safe [7]. No serious adverse effects appear in the controlled studies cited here.

No meaningful drug interactions are documented. Because it is taken for calming effects, stacking it with sedatives is the main thing to think about, though no study has shown harm.