An investigational drug is a medication that has not yet been approved for sale or prescription by the FDA. It is currently being tested in humans to determine whether it is safe and effective enough to earn that approval. You may also see it called an “experimental drug,” which means the same thing. Every prescription drug on the market today was once an investigational drug, and roughly 86% of those that enter human testing never make it through.
How a Drug Becomes “Investigational”
Before any new medication can be tested in people, its developer (called the sponsor) must file an Investigational New Drug (IND) application with the FDA. This is a legal requirement. Federal law prohibits transporting or distributing an unapproved drug across state lines, and the IND is essentially an exemption from that rule. Once the FDA accepts the application, the compound’s legal status changes: it officially becomes a “new drug” under federal law and enters the regulatory system.
The IND application is not a simple form. It must include three major categories of information. First, detailed chemistry, manufacturing, and control data describing exactly what the drug is made of, how it’s produced, and how the manufacturer ensures consistency and stability. Second, results from preclinical studies, meaning laboratory and animal testing that demonstrates the drug is reasonably safe enough to try in humans. This section requires an integrated summary of all toxic effects observed in animals and in lab tests. Third, a proposed plan for the clinical trials themselves, including how many people will be enrolled, what doses will be tested, and how safety will be monitored.
What Testing Looks Like
Once the IND is accepted, the drug moves through a series of human testing phases. Each phase has a different purpose, involves more people, and takes longer than the last.
In Phase 1, the drug is typically given to 20 to 100 volunteers, often healthy people. The goal is basic safety: researchers are looking for how the body absorbs, processes, and eliminates the drug, and what side effects appear at different doses. This phase usually lasts several months.
Phase 2 expands to up to several hundred people who actually have the disease or condition the drug is meant to treat. Now the focus shifts to effectiveness. Does the drug actually do what it’s supposed to do? Researchers also continue tracking side effects. This phase can last anywhere from several months to two years.
Phase 3 is the large-scale test. Between 300 and 3,000 patients with the target condition receive the drug, often compared against a placebo or an existing treatment. These trials run one to four years and generate the data the FDA will ultimately use to decide whether to approve the drug. Side effects that are rare enough to miss in smaller groups may show up here for the first time.
If the drug clears all three phases and the FDA grants approval, Phase 4 studies may follow. These involve thousands of patients and monitor for long-term safety issues that only emerge after widespread use.
Why Most Investigational Drugs Fail
A large-scale analysis of over 2,000 compounds and nearly 20,000 clinical trials conducted by 18 major pharmaceutical companies between 2006 and 2022 found that the average likelihood of a drug going from Phase 1 to FDA approval was 14.3%. Some companies managed rates as high as 23%, while others succeeded only about 8% of the time. The industry-wide benchmark from earlier analyses was even lower, around 10%. Drugs fail for many reasons: unexpected side effects, lack of meaningful benefit over existing treatments, or manufacturing problems that make consistent production impossible.
How Patients Are Protected
Because investigational drugs are unproven by definition, several layers of oversight exist to protect people who participate in trials. The most important is the Institutional Review Board (IRB), an independent committee that reviews every clinical trial before it begins and continues monitoring it throughout. The IRB’s job is to make sure the rights and welfare of participants are protected. It reviews not just the study design but the entire informed consent process: what information participants receive, how much time they’re given to consider it, who explains the risks to them, and whether their agreement is truly voluntary.
If the research team wants to change anything about the study protocol, the IRB must approve those changes before they take effect, with one exception: if an immediate change is needed to protect participants from a clear safety hazard. The IRB also has the authority to send observers to watch the consent process or the research itself if concerns arise.
On top of IRB oversight, federal regulations require specific labeling for investigational drugs. Every package must carry the statement: “Caution: New Drug, Limited by Federal law to investigational use.” The label cannot claim the drug is safe or effective, because that has not yet been established.
Getting an Investigational Drug Outside a Trial
Most people encounter investigational drugs by enrolling in a clinical trial. But two pathways exist for patients who can’t join a trial and have run out of approved options.
Expanded Access
The FDA’s expanded access program (sometimes called compassionate use) allows patients to receive an investigational drug when all of the following are true: they have a serious or life-threatening condition, no comparable approved treatment exists, enrolling in a clinical trial is not possible, the potential benefit justifies the risks, and providing the drug won’t interfere with the ongoing trials needed to bring it to market. The FDA reviews and must authorize each request.
Right to Try
The federal Right to Try Act, signed into law in 2018, created a separate pathway. It applies to patients diagnosed with a life-threatening disease who have exhausted all approved treatments and cannot participate in a clinical trial. A physician must certify these conditions in writing, and the physician cannot be compensated by the drug manufacturer for doing so. The patient or their legal representative must provide written informed consent. Unlike expanded access, Right to Try requests do not require FDA review or IRB approval. The FDA’s role is limited to receiving and posting information about these cases. However, drug manufacturers are not required to provide their investigational products through this pathway, and many do not.
What “Investigational” Actually Means for You
If you see a drug described as “investigational” in a news story, a clinical trial listing, or a conversation with your doctor, the key thing to understand is that it occupies a specific legal and scientific space. It has shown enough promise in lab and animal studies to be tested in humans, and it has passed an FDA review to begin that testing. But it has not yet proven that it works well enough, or safely enough, to be prescribed to the general public. The testing process is long, expensive, and most candidates don’t survive it. That doesn’t mean investigational drugs are dangerous or useless. It means the evidence isn’t complete yet, and the protections in place exist to make sure patients aren’t harmed while that evidence is gathered.