The Biologically active substance present in the any dosage form is known Active Pharmaceutical Ingredient or Bulk Drug. A dosage form of a drug is traditionally composed of two things: The API, which is the drug itself; and an excipient, which is the substance of the tablet, or the liquid the API is suspended in, or other material that is pharmaceutically inert. FDA's defines an API's as "any component that provides pharmacological activity or other. effect in the diagnosis, cure, mitigation, treatment or prevention of disease, or to affect. structure or any function of the body of man or animals.