Active Pharmaceutical Ingredient

API, drug substance, drug product

These are not interchangeable, and filings depend on the distinction:

• API / drug substance — the active molecule
• Excipients — everything else deliberately included (see excipient)
• Drug product — the finished dosage form the patient receives

A single API appears in many drug products. The API is manufactured, tested and released against its own specification before formulation begins, and in eCTD terms lives in Module 3.2.S while the drug product occupies 3.2.P.

GMP applies, differently

API manufacture is governed by ICH Q7, not by the finished-product regulations. The requirements tighten progressively: early synthetic steps are controlled more lightly, with full GMP applying from the introduction of the starting material onward. Deciding where GMP starts is a real regulatory judgement with real cost consequences, and it is negotiated with agencies rather than assumed.

The concentration problem

API manufacture is geographically concentrated, heavily in China and India, and often further concentrated at the level of individual intermediates. A drug can have several qualified finished-product sites and still depend on one plant for a key starting material.

This is the structural cause behind a large share of drug shortages: the shortage is rarely at the visible end of the chain. It is also why regulators have pushed for supply-chain transparency that most sponsors historically did not hold, because their supplier's supplier was never in scope.

Impurities travel downstream

An API's impurity profile is inherited by every product made from it. Genotoxic impurities are controlled under ICH M7, elemental impurities under Q3D, and the nitrosamine class prompted industry-wide reassessment after 2018. A change at the API step is never only an API change.