This is a common question, and the answer is: It depends. Some people with ADHD find that using L-theanine helps balance the effects of stimulant medications, but it's always best to get professional medical advice for your specific situation. It's not guaranteed to work the same way for everyone, and caffeine can have adverse effects when taking a stimulant.  Personally, I take a stimulant and enjoy one matcha green tea latte every day, and it instantly calms me.  As with any supplement, the effects of L-theanine can vary from person to person, and while it's generally considered safe, it's always a good idea to check with your doctor before starting any new supplement regimen.