I'll tell you a secret: oxygen is a drug. It's even used for anhesthesis. Oh, and you very body produces hormones that can alter your mind. Adrenaline, endorphines, and many others.
Doing sports for example activates a lot of these secretions, and that's why a lot of people are - more or less - addicted to their morning jog, evening tennis or what do I know.
You might not have used drugs the obvious way, but you sure have used mind-altering substances. Because you live. Sorry for that.