What Are L-Theanine Side Effects?

If you’ve ever tried a few different sleep supplements — and take it from me, I’ve tried a lot of them — you’re probably familiar with some common ingredients. Magnesium (in various forms), melatonin, valerian, tryptophan, and l-theanine are all pretty common on the back of ingredient labels. And with each comes different effects — and perhaps different side effects.

While I’ve never had much of a problem with sleep supplement side effects (apart from some working a little too well and leaving me drowsy), it’s worth becoming knowledgeable on these ingredients and how they may impact you.

Theanine Pill

L-theanine is a calming amino acid that’s non-essential to bodily function, and it’s also non-dietary, meaning you won’t get a lot of it in your day to day diet. As far as food goes, it’s found primarily in green tea, which could be one reason why green tea is so calming to drink. There’s also evidence that theanine, when paired with caffeine, can enhance focus and concentration better than caffeine alone, as well as reduce the likelihood of jitters.

Generally, l-theanine has relatively few reported side effects in the research space. It’s been called a “safe” and “natural” sleep aid, and it may help combat stress and even potentially spikes in blood pressure associated with stress. In some human studies, tea extracts containing theanine have caused dizziness and headaches in a few subjects, though it’s unclear which components in the extracts caused these.

My Experience with L-Theanine

I’ve never had an adverse reaction to theanine, though I generally keep any dosages around 100-200mg, depending on if I’m pairing it with caffeine. My suggestion? If you’re taking a sleep supplement with theanine, follow the manufacturer instructions. If you experience adverse reactions, talk to a medical professional (it may be a good idea to do this beforehand if you’re on prescription medications).

Some of my favorite sleep supplements contain theanine, including True Athlete’s ZMA formula and Onnit’s calming New Mood.


Having written hundreds of articles on SupplementYourSleep.com, Caitlyn knows what she's talking about when it comes to getting the best night's sleep. When not sleeping, reading about sleep, or writing about sleep, you can usually find her at the gym. Want to get in touch? The best way is to leave a comment on one of her articles.