A systematic review of nine randomized trials concluded that oral L-theanine may assist with stress or anxiety in people facing stressful conditions. The studies commonly examined 200 to 400 milligrams per day, but those research amounts are not personal dosing recommendations. The reviewers also said that larger and longer trials were needed before L-theanine could be justified as a therapeutic agent.

Direct placebo-controlled findings are less encouraging. In 80 young adults completing cognitive tests and a virtual-reality active-shooter drill, 200 milligrams of L-theanine did not improve salivary stress markers, heart rate, or state-anxiety scores. In a registered 28-day

The 2025 meta-analysis found a small improvement in choice reaction time during the first hour after L-theanine alone. L-theanine plus caffeine also improved selected vigilance and attention-switching accuracy measures during the first two hours. Other findings, including overall mood and one reaction-time result, were compatible with little or no difference. The result is best understood as a limited improvement on certain laboratory tasks.

Two small crossover trials add context. Among 24 overnight sleep-deprived adults, 200 milligrams of L-theanine reduced false alarms and improved the ability to separate relevant traffic scenes from distractions, but reaction-time improvement was not meaningfully different from placebo. In

Some studies infer relaxation from alpha brain-wave activity or stress questionnaires. Alpha activity is an electrical pattern associated with relaxed wakefulness, but it is not direct proof that someone feels calmer in daily life. These findings provide a plausible short-term calming signal, not evidence of durable relaxation or treatment of an anxiety disorder.

Sleep results do not show a simple benefit. In the 28-day trial, the authors reported improvement on a sleep questionnaire, but movement-based monitoring found less time asleep after 28 days and less light sleep than with placebo. A separate five-day trial reported

In the registered 28-day trial, 15 adults received a branded product providing 400 milligrams of L-theanine per day and 15 received placebo. Reported adverse events resolved by the end of the study or follow-up. Investigators judged changes in laboratory results and vital signs not clinically relevant. This offers limited short-term reassurance, but the sample was small and consisted of generally healthy adults.

The trial excluded people who were pregnant or breastfeeding, had certain sleep, stress, neuropsychiatric, or cognitive disorders, or used medicines and products that could affect the outcomes. It therefore cannot settle safety for those groups, medication interactions, or long-term

A systematic review of nine randomized trials concluded that oral L-theanine may assist with stress or anxiety in people facing stressful conditions. The studies commonly examined 200 to 400 milligrams per day, but those research amounts are not personal dosing recommendations. The reviewers also said that larger and longer trials were needed before L-theanine could be justified as a therapeutic agent.

Direct placebo-controlled findings are less encouraging. In 80 young adults completing cognitive tests and a virtual-reality active-shooter drill, 200 milligrams of L-theanine did not improve salivary stress markers, heart rate, or state-anxiety scores. In a registered 28-day

The 2025 meta-analysis found a small improvement in choice reaction time during the first hour after L-theanine alone. L-theanine plus caffeine also improved selected vigilance and attention-switching accuracy measures during the first two hours. Other findings, including overall mood and one reaction-time result, were compatible with little or no difference. The result is best understood as a limited improvement on certain laboratory tasks.

Two small crossover trials add context. Among 24 overnight sleep-deprived adults, 200 milligrams of L-theanine reduced false alarms and improved the ability to separate relevant traffic scenes from distractions, but reaction-time improvement was not meaningfully different from placebo. In

Some studies infer relaxation from alpha brain-wave activity or stress questionnaires. Alpha activity is an electrical pattern associated with relaxed wakefulness, but it is not direct proof that someone feels calmer in daily life. These findings provide a plausible short-term calming signal, not evidence of durable relaxation or treatment of an anxiety disorder.

Sleep results do not show a simple benefit. In the 28-day trial, the authors reported improvement on a sleep questionnaire, but movement-based monitoring found less time asleep after 28 days and less light sleep than with placebo. A separate five-day trial reported

In the registered 28-day trial, 15 adults received a branded product providing 400 milligrams of L-theanine per day and 15 received placebo. Reported adverse events resolved by the end of the study or follow-up. Investigators judged changes in laboratory results and vital signs not clinically relevant. This offers limited short-term reassurance, but the sample was small and consisted of generally healthy adults.

The trial excluded people who were pregnant or breastfeeding, had certain sleep, stress, neuropsychiatric, or cognitive disorders, or used medicines and products that could affect the outcomes. It therefore cannot settle safety for those groups, medication interactions, or long-term