The Drug The Drug is refer to any substance that brings about a change in biological function through it’s chemical actions.                                      OR The substance that act on living system at chemical( mollecular ) level.  Drug may be:      .  Synthesized in the body i.e harmones.      -  Or they may be chemicals not synthesized in the body i.e Xenobiotics ( both naturally and artificially prepared ).      -  Toxins are drugs as well.  Properties of Drugs   1. Physical properties of drug   2. Drug mollecular size   3. Drug reactivity and drug receptor bonding    4. Shape of the drug   5. Activation of prodrug   6. Drug nomenclature 1. Physical Nature of drug          > The drugs may be solid, liquid or gas at room temperature.      ...