L-theanine

What is L-theanine?

L-theanine is an amino acid found almost exclusively in tea leaves (Camellia sinensis) and in a few mushrooms. It is a big part of what gives green tea its savory, umami taste. Unlike the amino acids that build proteins, theanine is not essential in the diet. It crosses into the brain, where it is thought to influence neurotransmitters such as GABA, dopamine and serotonin and to increase alpha brain waves associated with relaxed alertness.

Theanine is often combined with caffeine, because many people find the pairing gives the alertness of caffeine with fewer jitters. Tea naturally contains both.

What the research says

A 2025 systematic review and meta-analysis in Nutrition Reviews examined randomized controlled trials of tea, theanine alone, or theanine plus caffeine in healthy people. It included 50 trials, 15 of which could be pooled. Compared with placebo, theanine plus caffeine produced small-to-moderate improvements in the first two hours after intake on measures such as choice reaction time, vigilance accuracy and attention switching, along with a possible lift in mood. Theanine alone showed a small improvement in reaction time. The confidence intervals were often wide, which means the size of the benefit is uncertain.

That supports the idea that theanine can make caffeine feel smoother and slightly sharpen focus in the short term. The evidence does not show that theanine burns fat, boosts metabolism or reduces appetite in a meaningful way.

We rate the evidence as moderate for short-term attention with caffeine, and limited or absent for weight-related claims.

How it’s used in supplements

L-theanine is sold on its own in 100 to 200 mg capsules, often for relaxation or sleep, and combined with caffeine in focus and energy products. Typical studied doses are about 100 to 200 mg, sometimes paired with 50 to 100 mg of caffeine. It is generally well tolerated in short-term studies, with occasional reports of headache or drowsiness.

Because theanine may lower blood pressure slightly, people taking blood pressure medication should check with their doctor. Pregnant and breastfeeding women should avoid concentrated theanine supplements because safety data are limited.

In Java Burn, L-theanine is listed for metabolism, reduced hunger and cognitive function, but the amount per packet is not disclosed. Its most plausible role is to soften the stimulant effect of the coffee and green tea it is mixed with.

If you want that effect, a cup of green tea or a single theanine capsule with a stated dose is a simple, inexpensive way to try it.