What Is L-Theanine? Benefits, Side Effects & More

L-theanine is a nutrient found in some plants and supplements. It has gained popularity for its potential health benefits, particularly related to stress relief and improved sleep. We spoke with registered dietitians to get the full scoop on L-theanine, from benefits and side effects, to who should take it and who should avoid it. 

What Is L-Theanine? 

L-theanine is a non-protein, water-soluble amino acid known for its potential to promote relaxation and ease stress without causing drowsiness. It is found naturally in plants like matcha, green, black and white teas, and fungi like some mushrooms. It is also available in supplement form, typically as a pill or powder. 

Potential Health Benefits of L-Theanine

While L-theanine is best known for helping people relax, it may have other potential health benefits as well.

Promotes Relaxation and Relieves Stress

According to Kelsey Costa, registered dietitian nutritionist and founder of Dietitian Insights, “L-theanine may have significant therapeutic effects on mental health, such as reduced depression and stress and improved emotional and cognitive function.”

Studies show that L-theanine helps lower stress-related symptoms, improve sleep quality and promote cognitive function in both healthy individuals and those with major depressive disorders. Athletes and other individuals under stress have also reported improved performance and reduced stress responses when using L-theanine.

May Increase Focus and Attention

L-theanine may enhance attention, focus and cognitive performance. It may be even more effective when combined with caffeine, which can help improve memory and executive function, and reduce distractions and mind wandering. L-theanine might also be beneficial for people with ADHD because it supports sustained attention and impulse control. However, more research on this topic is needed to clarify the relationship, especially for kids and young adults. 

Supports the Immune System 

L-theanine has antioxidant, immunoregulatory and anti-inflammatory properties. Studies suggest it may decrease the instances of respiratory infections, aid in flu prevention and decrease inflammation in the digestive tract.

“It’s important to note that many of the available studies on potential health benefits have small sample sizes, show limited improvements and often involve healthy individuals rather than those with chronic health problems,” says Emily Haddock, RD, LDN, registered dietitian and owner of Music City Nutrition & Wellness. Additional research is needed on more diverse populations to further clarify and support these benefits. 

Potential Side Effects of Taking L-Theanine

“With the small bit of research currently available, there aren’t many confirmed side effects or direct risks associated with taking L-theanine,” Gabbie Ricky, M.S., RDN, performance nutrition specialist, tells EatingWell. 

“As with any supplements you’re considering, however, make sure you purchase from a reputable manufacturer. Look for supplements that have the Good Manufacturing Practices (GMP) certification and have been rigorously third-party tested for product quality and purity,” she adds.

Some side effects associated with L-theanine are often due to the overconsumption of caffeine by those who increase L-theanine intake by drinking tea. These side effects may include:

• Headaches
• Nausea
• Jitteriness
• Irritability. 

The recommended daily intake of caffeine is 400 milligrams or less.

Who Should Take L-Theanine? 

“People who regularly work in high-risk environments or under high levels of stress may benefit from the potential cognitive benefits of L-theanine, such as improved focus especially when paired with caffeine,” says Ricky. “Some examples of these populations might include professional athletes, tactical athletes (like firefighters, military personnel, etc.) and healthcare workers,” she adds.

People with poor sleep quality, depression and anxiety might also benefit from L-theanine, but it’s important to talk with a healthcare provider before adding any new supplements to your routine. 

Who Should Avoid It?

While L-theanine is safe for most people, some should avoid it. “People who are pregnant or lactating should avoid L-theanine as it has not been adequately tested in these populations,” adds Haddock.

People who take medications for high blood pressure may need to avoid L-theanine, as it may further reduce blood pressure.

Also, anyone receiving chemotherapy for cancer treatment should avoid L-theanine supplements until after their treatment course is complete. “In supplemental doses, the potent antioxidant function of L-theanine may reduce the efficacy of certain chemotherapy drugs,” Ricky tells EatingWell.

It’s always recommended to speak with a healthcare provider before adding any new supplements to your routine, particularly if you are undergoing treatment for cancer or high blood pressure.  

The Bottom Line

L-theanine is a non-protein amino acid that is found in some foods like tea and mushrooms. It is also available in supplement form. Many people may be able to safely take L-theanine without negative side effects, but some people (like those who are pregnant or undergoing cancer treatment) should avoid it. While L-theanine shows promise of potential health benefits, more extensive research is needed on larger and more diverse populations to confirm this.