Everything is a drug. Drugs are just some particular classification of influence, and somewhat of a vague one at that. Drugs effect the physiochemistry of the person who takes them.
Drugs have their own particular effects on the functioning of the nervous system.
This is not only true of drugs. This is true of all influences that a human deals with. People have evolved to respond to certain drugs in certain ways. Evolved to have certain chemical reactions, often times resulting in certain physical or mental actions, as is the way the body naturally functions. Oxygen effects the physiochemistry of the person who breaths it.