The main drawback is that the specific effects of L-theanine have only been tested in a small number of randomized controlled trials which predominantly included healthy people without chronic conditions. Therefore, further high-quality studies are needed to bolster the current evidence.
The clinical evidence cited above shows that supplementation with L-theanine is well-tolerated, with no known reports of adverse effects or toxicity. In rodents, L-theanine has remarkably low toxicity, with one study failing to find any toxic effects when administering up to 4,000 mg of L-theanine per kg of body weight daily for 13 weeks.[1] Additional animal studies have reported similar results with no observed toxicity at remarkably high doses.[2] Consequently, it is generally assumed that L-theanine is safe for humans to consume,[2] particularly given its long history of consumption in tea. However, specific safety studies in humans are lacking.