L-theanine is generally well tolerated, with clinical trials consistently reporting mild or no side effects. But certain groups face real risks from supplementing with it, primarily because of its effects on blood pressure, its calming properties, and the lack of safety data for vulnerable populations.
Pregnant and Breastfeeding Women
There is essentially no clinical data on L-theanine supplementation during pregnancy or breastfeeding. The NIH’s LactMed database, which tracks drug safety during nursing, reports that no published information exists on maternal levels, infant levels, effects in breastfed infants, or effects on breast milk production. The small amounts naturally present in green tea are likely fine during nursing, but the concentrated doses found in supplements (typically 100 to 400 mg) are a different matter entirely.
L-theanine has a short half-life of about one hour, meaning it clears from breast milk within three to five hours. Even so, the NIH specifically advises avoiding L-theanine supplements while nursing a newborn or preterm infant. Without human safety studies in pregnant women, the same caution applies throughout pregnancy.
People With Low Blood Pressure
L-theanine can lower blood pressure in a dose-dependent way. If your blood pressure already runs low, supplementing could push it further down, causing dizziness, lightheadedness, or fainting. This effect is mild for most people, but it becomes a genuine concern in two situations: if you have a history of hypotension, or if you take blood pressure medication.
Combining L-theanine with antihypertensive drugs creates a risk of blood pressure dropping too low. If you’re on medication for high blood pressure and want to try L-theanine, monitoring your blood pressure closely is essential. The interaction isn’t guaranteed, but the combined effect can be unpredictable.
People Scheduled for Surgery
Because of its blood pressure-lowering effect, L-theanine can complicate anesthesia and blood pressure management during surgical procedures. The Mayo Clinic Proceedings recommends stopping L-theanine at least 24 hours before surgery. This is a shorter hold period than many supplements require, but it’s still important to follow. If you’re preparing for any procedure involving anesthesia, let your surgical team know you take it.
Children Without Medical Guidance
L-theanine has been studied in children, mostly boys aged 8 to 15 with ADHD. A clinical trial established the safety of 400 mg per day (split into two doses) over six weeks in boys aged 8 to 12. Side effects were minimal, with one notable exception: one child developed facial tics after four days of treatment, which resolved after stopping the supplement. No other serious adverse effects were reported across multiple pediatric studies.
That said, children aren’t small adults when it comes to supplements. Dosing in these trials was carefully weight-based (around 2.5 to 3.0 mg per kilogram of body weight), not the flat 200 mg capsules sold in stores. Parents giving children L-theanine without adjusting for weight or understanding the limited scope of safety data are working with very little margin.
People Taking Sedatives or Sleep Medications
L-theanine promotes relaxation and can cause drowsiness, particularly at higher doses. While formal interaction databases haven’t flagged direct conflicts with common sedatives like benzodiazepines, the calming effects can theoretically stack. If you already take something that makes you sleepy, whether a prescription sedative, an antihistamine, or even alcohol, adding L-theanine on top could amplify that drowsiness. This matters most for tasks requiring alertness, like driving.
What Side Effects Look Like
For people who don’t fall into the groups above, L-theanine has a notably mild side effect profile. A 2024 systematic review of L-theanine in patients with mental health conditions found that the most common complaints were nausea, abdominal pain, headache, irritability, and occasional diarrhea or constipation. None of these were serious, and in several trials, the side effect rates were statistically identical between L-theanine and placebo groups. Multiple studies reported zero adverse effects at all.
The FDA reviewed L-theanine as a food ingredient and had “no questions” about its safety at doses up to 250 mg per serving, granting it GRAS (Generally Recognized as Safe) status. This doesn’t mean higher doses are dangerous, but it does mark the threshold that received formal regulatory review. Most supplements on the market contain 100 to 200 mg per capsule, which falls within this range.
The Bottom Line on Risk
L-theanine is one of the better-tolerated supplements available. The people who should genuinely avoid it are pregnant or breastfeeding women (due to absent safety data), anyone with low blood pressure or on blood pressure medication (due to additive effects), and anyone approaching surgery (due to anesthesia complications). For everyone else, the risk profile is low, but it’s still a supplement with real physiological effects, not just a harmless amino acid from tea.