A drug may be defined as a substance which modifies the functions of the
body or of some organ without sensibly imparting nourishment. This action
may be one of stimulation or of depression. A drug is taken for its
medicinal action, a food adjunct for its modifying action on food. It is
impossible to give a quite satisfactory definition, or to draw sharp
distinctions. For example, tea, coffee, alcohol and tobacco are sometimes
placed in one group, and sometimes in another, according to opinion of
their action and the definition of the terms food adjuncts, drugs and
poisons. The difference of grouping often depends upon intensity rather
than of kind of action. If taken frequently and not in quantity sufficient
to have a markedly medicinal action, such things are generally called food
adjuncts or supplementary foods, although much may be said in favour of a
different view. The volatile oils of mustard, caraway, cloves, etc., are
used in medicine; also the alkaloids of coffee and cocoa. Even honey is
used as a mild laxative for infants; that is, as a drug. The difference
between a drug and a poison is one only of degree. Some of the most
esteemed drugs have to be administered in very small quantities, or they
cause death; e.g., strychnine and morphine.

Classifications are necessary for methodical study, and for assisting the
memory in grasping large numbers of things which can be grouped together.
Classifications, however, are artificial, not due to natural lines of
demarkation, but according to man's knowledge and convenience; hence a
group is apt to approach and finally merge into another group, although on
first consideration they appeared quite distinct. The disregard of this
often leads to confusion and useless discussions.

Plants, like animals, as the result of tissue change, have certain used-up
or waste matters to get out of the way. Animals have special excretory