An international nonproprietary name (INN) is an official generic and nonproprietary name given to a pharmaceutical substance or an active ingredient, encompassing compounds, peptides and low-molecular-weight proteins (e.g., insulin, hormones, cytokines), as well as complex biological products, such as those used for gene therapy. INNs are intended to make communication more precise by providing a unique standard name for each active ingredient, to avoid prescribing errors. The INN system was initiated by the World Health Organization (WHO) in 1953.

Having unambiguous standard names for each pharmaceutical substance (standardization of drug nomenclature) is important because a drug may be sold under many different brand names, or a branded medication may contain more than one drug. For example, the branded medications Celexa, Celapram and Citrol all contain the same active ingredient whose INN is citalopram. The antibacterial medication known as co-trimoxazole as well as those under the brand names Bactrim and Septran all contain two active ingredients easily recognisable by their INN: trimethoprim and sulfamethoxazole.

The WHO publishes INNs in English, Latin, French, Russian, Spanish, Arabic, and Chinese, and a drug's INNs are often cognate across most or all of the languages, with minor spelling or pronunciation differences, for example: paracetamol (en) paracetamolum (la), paracétamol (fr) and парацетамол (ru). An established INN is known as a recommended INN (rINN), while a name that is still being considered is called a proposed INN (pINN).

National nonproprietary names such as British Approved Names (BAN), Dénominations Communes Françaises (DCF), Japanese Adopted Names (JAN) and United States Adopted Names (USAN) are nowadays, with rare exceptions, identical to the INN.

Mandate The World Health Organization has a constitutional mandate to "develop, establish and promote international standards with respect to biological, pharmaceutical and similar products". The World Health Organization collaborates closely with INN experts and national nomenclature committees to select a single name of worldwide acceptability for each active substance that is to be marketed as a pharmaceutical. To avoid confusion, which could jeopardize the safety of patients, trade-marks should neither be derived from INNs nor contain common stems used in INNs.

Each drug's INN is unique but may contain a stem that is shared with other drugs of the same class. In this context, a stem is a syllable (or syllables) created to evoke in the name the pharmacological mechanism of action or the chemical structure of the substance. Stems are mostly placed word-finally (suffixes), but in some cases word-initial stems (prefixes) are used. For example, the beta blocker drugs propranolol and atenolol share the stem -olol (as a suffix), and the benzodiazepine drugs lorazepam and diazepam share the stem -azepam (also a suffix) The list of stems in use are collected in a publication informally known as the Stem Book.

Some examples of stems are:

The School of INN is a WHO International Nonproprietary Name Programme initiative launched in 2019, which aims to provide information to pharmacy, medical and health students, as well as health professionals and other stakeholders on how an INN is designed and constructed.