Drugs work because your body has the ability to react to them. If your body can react to drugs is, usually, because there are internal receptors that can bind to them. But of course naturally occurring concentrations of 'drug'-like components are much lower and extended over time than a pill, and the components themselves are milder than their drugstore counterparts (except counted instances such as norepinephrines). Besides the fact that generating them requires (usually) exercise (for example, for endorphines and melatonin/cortisol/tyroxine fine-tunning), and/or yoga-style body control.

A report from your friendly biomedical biologists.