According to the Food and Drug Administration (FDA), investigational new drug means a new drug or biological drug that is used in a clinical investigation. The term also includes a biological product that is used in vitro for diagnostic purposes. The terms "investigational drug" and "investigational new drug" are deemed to be synonymous. (21 CFR 312.3)

• The use of an investigational drug (IND) in human subjects requires approval by the FDA and the IRB.
• An IND number is generally required for a drug (including a drug with marketing authorization) if it is intended to:
• Support a new indication for use, establish safety or efficacy of the drug, support a change in the approved route of administration (including method of assembly) or dosage level;
• Support a change in the approved patient population (e.g. pediatric vs. adults) or a population at greater/increased risk (e.g. immunocompromised, elderly, etc.);
• Support a change in the promotion/ advertising/ labeling/ packaging of an approved drug.
• An IND number may also be required for investigations that attempt to gain further information about an approved use. The study sponsor and/or investigator cannot represent (in a promotional context) that the drug is safe and effective for the purposes in which it is under investigation.