Active Pharmaceutical Ingredient (API) is the term used to refer to the biologically active component of a drug product i.e. tablet, capsule. Generally, drug products are included with various components. Among them, API is the primary ingredient. Other ingredients in drug products are known as 'excipients'.

These substances, which make up a variable fraction of the drug product, are required to be biologically safe. APIs are used for pharmacological activity in the diagnosis, cure, mitigation, treatment, or prevention of disease or to affect the structure or function of the body.