Learn how clinical trials progress through safety testing to market approval.
Before a drug ever reaches a human, it goes through years of laboratory and animal testing called preclinical research. Scientists study the compound in cell cultures and animal models to understand how it behaves in living systems, whether it appears safe, and whether it shows potential to treat the target disease. This process alone typically takes 3 to 6 years and is funded by pharmaceutical companies, biotech firms, or academic research institutions.
Once preclinical data shows enough promise, the drug sponsor files an Investigational New Drug (IND) application with the FDA. The IND includes all preclinical data, a proposed plan for human testing, and detailed manufacturing information. The FDA has 30 days to review and either approve the application or place a clinical hold. If approved, human trials can begin.
From there, clinical trials move through a series of phases. Each phase asks a different question, enrolls more people, and builds on the data gathered in the previous phase. The entire journey from lab to approved drug takes an average of 10 to 15 years and costs hundreds of millions to billions of dollars.
Could this compound work?
Laboratory and animal testing to evaluate safety, biological activity, and formulation. Researchers identify a target (like a protein or gene pathway), develop a compound that interacts with it, and test it in cell cultures and animal models. Preclinical studies also determine how the drug is absorbed, metabolized, and excreted.
Only about 1 in 1,000 compounds tested in the lab will ever reach human trials.
Is it safe for humans?
First-in-human testing with a small group. Researchers start with very low doses and gradually increase them (dose escalation) to find the highest dose that doesn't cause unacceptable side effects. They closely monitor how the drug is absorbed, distributed, metabolized, and eliminated by the body (pharmacokinetics).
Participants are typically monitored very closely, often in a clinical research facility. Phase 1 trials are not designed to test whether the drug works—only whether it's safe enough to continue testing.
Does it actually work?
The first real test of effectiveness. Participants who have the target disease receive the drug, and researchers measure whether it produces the intended therapeutic effect. Phase 2 also continues evaluating safety and helps determine the optimal dose for Phase 3. Many Phase 2 trials are randomized and some include a placebo or active comparator group.
This is where many drugs fail. A compound might be safe but simply not effective enough to justify the cost and risk of a large Phase 3 trial. For rare diseases, Phase 2 trials may be smaller due to limited patient populations.
Is it better than what already exists?
Large-scale, definitive trials that compare the new treatment against the current standard of care or a placebo. These are the trials the FDA relies on most heavily when deciding whether to approve a drug. Phase 3 trials are almost always randomized and usually double-blinded. They measure clearly defined primary endpoints (like survival time, tumor shrinkage, or symptom improvement) and track a comprehensive list of side effects.
Phase 3 trials are extremely expensive, often costing tens or hundreds of millions of dollars. If a Phase 3 trial succeeds, the sponsor submits a New Drug Application (NDA) or Biologics License Application (BLA) to the FDA for review.
Should this drug be approved?
After successful Phase 3 trials, the drug sponsor submits all clinical data to the FDA. An NDA or BLA includes results from all trial phases, proposed labeling, manufacturing details, and safety data. The FDA review team evaluates the evidence and may convene an advisory committee of independent experts. Standard review takes about 10 months; priority review (for drugs that offer major advances) takes about 6 months.
The FDA can approve the drug, request additional data, or deny approval. For rare diseases, the FDA has special expedited pathways: Fast Track, Breakthrough Therapy, Accelerated Approval, and Priority Review.
What happens long-term?
Post-market surveillance studies conducted after a drug is approved and available to patients. These studies monitor long-term safety in much larger and more diverse populations than the clinical trials. Phase 4 studies can also explore new uses for the drug, test it in different patient populations, or study drug interactions.
Phase 4 data has led to drugs being pulled from the market years after approval when rare but serious side effects emerged. Vioxx is a well-known example—it was withdrawn in 2004 after post-market data showed increased cardiovascular risk.
About 90% of drugs that enter Phase 1 testing never make it to FDA approval. That's not a failure of the system—it's the system doing exactly what it's designed to do. Clinical trials exist to separate treatments that actually work from ones that don't, and to catch safety issues before a drug reaches millions of patients. The phases function as a series of increasingly rigorous filters.
Phase 3 trials are the most critical hurdle. They're large, expensive, and they're what the FDA ultimately relies on to decide whether a drug should be approved. If a trial you're considering is in Phase 3, that means the treatment has already cleared safety testing and shown signs of effectiveness in earlier phases. That's meaningful.
Success rates vary significantly by disease area. Oncology drugs have historically had lower Phase 3 success rates (around 30–40%), while drugs for infectious diseases and hematology tend to have higher rates. Rare disease drugs have benefited from regulatory incentives like the Orphan Drug Act, which provides tax credits, extended market exclusivity, and fee waivers to encourage development.
The FDA recognizes that rare diseases face unique development challenges—small patient populations, limited natural history data, and fewer commercial incentives. To address this, several expedited programs exist that can significantly speed up the approval timeline.
Fast Track designation is for drugs that treat serious conditions and fill an unmet medical need. It allows more frequent meetings with the FDA and eligibility for rolling review (the FDA can review sections of the application as they're submitted, rather than waiting for the complete package).
Breakthrough Therapy designation is for drugs that show substantial improvement over existing treatments based on early clinical evidence. This provides all Fast Track features plus more intensive FDA guidance on efficient trial design.
Accelerated Approval allows the FDA to approve a drug based on a surrogate endpoint—a lab measurement or physical sign that is reasonably likely to predict clinical benefit, even if the final clinical outcome hasn't been measured yet. The drug can reach patients faster, but the sponsor must conduct confirmatory trials afterward.
Priority Review shortens the FDA review timeline from the standard 10 months to 6 months. It's granted to drugs that would be significant improvements in safety or effectiveness compared to available therapies.
If you're looking at a Phase 1 trial, you're among the first humans to receive this drug. The focus is on safety, not effectiveness. You may be helping establish the right dose for future patients. These trials involve the most uncertainty but also represent the earliest access to completely new treatments.
Phase 2 trials are where you start to see whether the drug might actually help your condition. If a Phase 2 trial for your disease is recruiting, it means the drug has already demonstrated a reasonable safety profile. You'll be closely monitored, and researchers will be measuring specific outcomes related to your disease.
Phase 3 trials offer the most data about what to expect. By this point, researchers know the drug is reasonably safe and have evidence it works. Phase 3 compares it against the current best treatment (or placebo if no standard exists). These trials typically have the most structured protocols and the largest support teams.
Phase 4 trials are conducted after a drug is already approved and available. If you're in a Phase 4 trial, you're receiving a drug that has full FDA approval. These studies help researchers understand how the drug performs in broader, real-world populations.