A surprising amount of foodstuffs can be classified as a drug. Not only does tea contain caffeine (in small quantities), but it also containes an amino acid, L-Theanine, which can cross the blood brain barrier and has psychoactive properties.[1] Some people like the effect enough to supplement it.[2]

[1]: http://en.wikipedia.org/wiki/Theanine#Effects

[2]: http://www.amazon.com/Now-Foods-L-Theanine-Veg-Capsules-60-C...