The finding concerned a brief stress response
Twelve men completed counterbalanced sessions using 200 mg of L-theanine in water, placebo or no treatment around a mental-arithmetic task. The study reported a smaller heart-rate response with theanine relative to placebo. It did not study long-term treatment of a heart condition. [1]
The proposed mechanism was not directly established
The authors interpreted heart-rate variability as consistent with reduced sympathetic activation. They did not measure the suggested brain neurotransmitter changes, and their small male-only sample limits generalization. Two authors were affiliated with the ingredient manufacturer. [1]
A calmer feeling does not diagnose the cause
Heart rate can change for many reasons. Caffeine-containing blends may have different effects from pure theanine, and a supplement should not be used to compensate for stimulant-related symptoms or replace prescribed heart treatment. [2][3]
Use symptoms to decide urgency
Imagine recurrent palpitations are the reason for taking theanine. Arrange assessment rather than adjusting a supplement dose to chase the pulse. Palpitations with chest pain, breathlessness or fainting need urgent medical help. [4]
| Feature | Limit |
|---|---|
| Twelve men | Small selected sample |
| Mental-arithmetic stress | Not resting-rate treatment |
| Single 200 mg exposure | Not a chronic regimen |
| Persistent palpitations | Need clinical assessment |
Sources for this article
- L-Theanine reduces psychological and physiological stress responses.
Source consulted: 2026-09-21. This is a reference resource, not a claim that we conducted the studies or tested a product. - Spilling the Beans: How Much Caffeine is Too Much? | FDA
Source consulted: 2026-09-21. This is a reference resource, not a claim that we conducted the studies or tested a product. - L-Theanine | Memorial Sloan Kettering Cancer Center
Source consulted: 2026-09-21. This is a reference resource, not a claim that we conducted the studies or tested a product. - Heart palpitations - NHS
Source consulted: 2026-09-21. This is a reference resource, not a claim that we conducted the studies or tested a product.