L-Theanine Side Effects
At this time, there are no known side effects of L-theanine. Although no problems have been reported in case reports or clinical studies, it does not mean that side effects are not possible. If you experience any reactions to L-theanine, make sure to notify your healthcare provider, as he or she may want to report these problems.
L-theanine (also known as just theanine) is an amino acid found in tea and is used in dietary supplements. There are no known L-theanine side effects. However, this simply means that no side effects have been reported, either in clinical studies or in case reports (individual reports from healthcare providers about side effects seen in their patients).
Most likely, this reflects a lack of studies large enough to find side effects; it probably does not indicate that L-theanine really is free of side effects. Animal studies suggest that L-theanine probably does not cause significant side effects in rats. In one study, increased kidney tumors were seen in rats exposed to L-theanine, although the details of the study suggest that the tumors were probably not related to this substance.
Make sure to let your healthcare provider know if you develop any side effects that you think may be related to L-theanine. Your healthcare provider will likely be interested in such possible side effects and may even want to report them, especially if they are particularly interesting or serious.
You should also let your healthcare provider know if you develop something that "just does not seem right" during treatment with L-theanine. While it may not be a side effect, your healthcare provider will be able to diagnose and treat the problem.