active pharmaceutical ingredient (API)

In any medicine, the active pharmaceutical ingredient, or API, is the molecule that actually does the work — the part that lowers your fever, kills the bacteria or relieves the pain. Everything else in the product is there to support, protect or deliver it.

The API is the drug substance in its pure chemical form, before it is mixed with anything else. Its physical and chemical properties — solubility, particle size, crystal form, stability, acidity — strongly shape how the final product must be designed. A poorly soluble API, for instance, may need micronization or a salt form to dissolve fast enough to be absorbed.

It is worth distinguishing the API from the finished medicinal product. A tablet labelled as containing 500 mg of a drug contains 500 mg of API plus a larger mass of excipients. Regulators set strict purity and quality standards for the API itself, separate from those for the finished product.