Theanine Side Effects
Although no problems have been reported by people using theanine, side effects are possible. This most likely reflects a lack of theanine studies large enough to detect side effects; it probably does not mean that the supplement is actually free of side effects. If you develop any theanine side effects, notify your healthcare provider so he or she can recommend treatment and report these findings.
Theanine (also known as L-theanine) is an amino acid found in tea and is used in dietary supplements. There are no known theanine side effects. However, this simply means that no side effects of theanine 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 theanine studies large enough to find side effects; it probably does not indicate that theanine really is free of side effects. Studies in rats suggest that theanine probably does not cause significant side effects in rats. In one study, increased kidney tumors were seen in rats exposed to theanine, although the details of the study suggest that the tumors were probably not related to theanine.
Make sure to let your healthcare provider know if you develop any side effects that you think may be related to theanine. Your healthcare provider will likely be interested in such possible side effects and may even want to report such side effects (especially if they are particularly interesting or serious). You should also let him or her know if you develop something that "just does not seem right" during treatment with theanine. While it may not be a side effect of theanine, your healthcare provider will be able to diagnose and treat the problem.