IMP stands for investigational medicinal product. It refers to any medicinal product being tested, used as a reference, or used as a placebo in a clinical trial. The term comes from EU regulatory language, specifically EU Regulation No. 536/2014, and it covers more ground than most people expect. An IMP isn’t just the experimental drug being studied. It also includes the placebo given to a control group and any already-approved medication used as a comparator.
What Counts as an IMP
The formal definition is broad: an IMP is “a medicinal product which is being tested or used as a reference, including as a placebo, in a clinical trial.” That means a drug already on the market with full regulatory approval still qualifies as an IMP if it’s being used in a trial as the test product, a reference product, or a placebo. The classification depends on how the product is being used in the trial, not on whether it’s new or approved.
In practice, a clinical trial protocol might compare an experimental drug plus standard background treatment against a placebo plus background treatment. In that design, both the experimental drug and the placebo are IMPs. The background treatment, however, may not be, depending on its role in the trial hypothesis.
IMP vs. Non-Investigational Medicinal Product
Clinical trials often involve medications that aren’t the focus of the study but are still part of the protocol. These are called non-investigational medicinal products, or NIMPs. A NIMP is any medicinal product used in a trial but not as an IMP. The most common example is background therapy that patients continue taking as part of their standard care.
The distinction hinges on the trial’s hypothesis. If a standard-of-care treatment is part of what the trial is investigating (for example, if the trial is studying how the experimental drug interacts with that treatment, or using it as a direct comparator), it becomes an IMP. If it’s simply maintained as a patient’s ongoing therapy and isn’t being evaluated, it stays a NIMP. This classification matters because IMPs face stricter manufacturing, labeling, storage, and reporting requirements.
How IMPs Are Manufactured and Released
IMPs must be produced under good manufacturing practice (GMP) standards, just like commercially available drugs. Before any IMP batch can ship to a trial site, a Qualified Person (QP) must formally certify it. The QP is a legally designated individual who reviews the manufacturing records, confirms compliance with the clinical trial authorization, and signs off that the batch meets all required specifications.
This certification must happen before the IMP is shipped to any site that doesn’t hold its own manufacturing authorization. If the trial sponsor submits amendments to the trial documentation, the QP needs access to the correct, updated versions before certifying any new batches. The QP is personally accountable to regulatory authorities for every certification decision they make.
Tracking IMPs at Trial Sites
Every trial site is required to maintain detailed drug accountability records for IMPs. These logs track every unit from the moment it arrives at the site through dispensing to individual participants and, if applicable, return or destruction. A typical accountability log records the date, subject ID, dose, quantity dispensed or received, running balance, lot number, and the initials of the person making the entry.
This level of tracking serves two purposes. It confirms that participants actually received the correct product at the correct dose, and it creates a verifiable chain of custody that regulators can audit. Discrepancies in accountability logs, such as missing units or balance errors, are red flags during inspections.
Storage and Temperature Requirements
Many IMPs require strict temperature control, and trial sites must continuously monitor storage conditions. When a temperature excursion occurs (the storage environment goes outside the acceptable range), the event must be documented and reported to the manufacturer. The manufacturer then conducts a risk analysis using stability data, often comparing the affected product to a retained control sample using validated testing methods.
Accelerated stability studies and freeze-thaw data help determine whether the excursion compromised product quality. If the analysis shows degradation or increased impurities, the affected stock may need to be quarantined or destroyed. Sites are expected to have written agreements with manufacturers that spell out exactly who is responsible for reporting excursions and how quickly it must happen.
Safety Reporting for IMPs
When a participant in a clinical trial experiences a serious and unexpected adverse reaction linked to an IMP, the trial sponsor must report it to regulators on a tight timeline. In the U.S., the FDA requires a written safety report within 15 calendar days of the sponsor first learning about the event. If the reaction is fatal or life-threatening, that window shrinks to 7 calendar days by phone or fax, with a follow-up written report.
In the EU, these events are called suspected unexpected serious adverse reactions (SUSARs), and they must be reported through the EudraVigilance database. For trials involving more than one IMP, sponsors can submit a single annual safety report covering all IMPs in the trial through the EU’s Clinical Trials Information System.
If the trial is blinded (meaning neither the participant nor the investigator knows who received the active drug versus placebo), sponsors should only unblind the specific participant who experienced the event. This preserves the integrity of the trial while still allowing the reaction to be properly evaluated.
The Investigator’s Brochure
Every IMP comes with a document called the Investigator’s Brochure, or IB, which gives trial investigators the information they need to understand the product’s safety profile. The structure follows international guidelines (ICH E6) and includes sections on the drug’s physical and chemical properties, results from animal studies covering pharmacology, metabolism, and toxicology, and all available data on effects in humans, including pharmacokinetics, safety, efficacy, and any marketing experience if the drug is already approved elsewhere.
The IB is a living document. It gets updated as new data emerges during the trial, and investigators are expected to review each version. The final section summarizes all available evidence and provides practical guidance for the investigator on risks to watch for, helping them make informed decisions about participant safety throughout the trial.