A drug is any substance or mixture of substances manufactured, sold or represented for use in
  • the diagnosis, treatment, mitigation or prevention of a disease, disorder, abnormal physical state, or symptoms thereof, in man or animal
  • restoring, correcting or modifying organic functions in man or animal.
Other definitions include, "anything that enters into the body and affects its functioning or some part of it in some way". "A drug, affects the body, mind or perceptions and changes the way a body works".

Although an apple changes the body by providing nutrients, these nutrients are required to function correctly, and do not alter in any large way the body's metabolism. A drug would, whilst active, change some function of the body (for better or worse), provide a perception shift etc. A drug is effectively a non-essential product that the body takes in for its own good, or that the body takes in for the mind's pleasure/expansion.

The difference between food and drugs is quite clear. Some foods contain drugs, or are also drugs. For example,chocolate.