How clinical trials work comes down to a staged filter: a new drug passes through three phases of human testing before the FDA reviews the data and, if it approves, the drug reaches pharmacy shelves. FDA's patient materials put Phase 1 at 20 to 100 volunteers and Phase 3 at 300 to 3,000.
The distance from concept to shelf runs long in most cases. Wikipedia's summary of drug development states that the entire process, from laboratory concept through Phase 1 to 3 trials to an approved drug, typically takes more than a decade. Most candidates never arrive. A study covering the 1980s and 1990s found that only 21.5% of candidates starting Phase 1 trials were eventually approved for marketing. From 2006 to 2015, the success rate from Phase 1 through successful Phase 3 trials was under 10% on average.
What happens before human testing?
Developers start with discovery, then preclinical work on microorganisms and animals. Under the process Wikipedia describes, an investigational new drug filing with the FDA is what allows human trials to begin. The FDA's review materials treat that early data as the base of the file. A drug developer must include everything about a drug, from preclinical data to Phase 3 trial data, in the application. For related coverage, see Work fatigue costs employers $218 billion a year, and health care carries outsized risk, federal data show.
What happens in each phase?
Each phase has a fixed job, and FDA's patient page on clinical research sets the sizes and durations.
- Phase 1 runs on 20 to 100 healthy volunteers for several months, checking safety and dosage.
- Phase 2 enrolls up to several hundred patients for several months to 2 years, testing efficacy and side effects.
- Phase 3 involves 300 to 3,000 volunteers with the condition, lasts 1 to 4 years, and produces most of the safety data.
- Progress is not automatic: FDA counts about 70% of drugs moving on after Phase 1 and about 33% after Phase 2.
The early phases also shape design. FDA notes that Phase 2 studies are not large enough to show whether a drug will be beneficial, so researchers use them to refine questions and build Phase 3 protocols. Cancer drugs run differently at the start: Phase 1 studies for cancer go to patients with that diagnosis rather than healthy volunteers.
Those drop-off numbers explain why failure is a normal outcome in the pipeline. A drug can be safe at low doses and still wash out at Phase 2 or Phase 3, once real patients and longer exposure enter the picture.
How long does the FDA review take?
Testing ends with a file, not a form. Once FDA receives a New Drug Application, the review team first decides whether it is complete, and it can refuse to file an incomplete package. If it is complete, FDA's materials give the review team 6 to 10 months to decide whether to approve the drug. Monitoring does not stop at approval. The same FDA pipeline lists post-market drug safety monitoring as its step 5.
Why does the whole process take so long?
The clock is dominated by evidence. Each phase answers one question at a time, from safe dosage to real-world benefit, and each answer has to hold up across hundreds or thousands of participants before the file moves. Attrition drives restarts, and restarts cost money. The two cost studies cited in Wikipedia's summary put pre-approval capitalized cost estimates at $2.6 billion and $1.1 billion respectively, which is why developers sequence decisions carefully before each phase. We covered a connected angle in Brooklyn's Maimonides moves to join NYC Health + Hospitals in a $2.2 billion New York hospital merger.
Conclusion: The bottom line
How clinical trials work, in short: three phases of testing in people, a 6 to 10 month FDA review, and a road of a decade or more where most candidates drop out. FDA's own numbers, 20 to 100 volunteers at the start and 300 to 3,000 at the end, show the scale at each step. The practical next step for anyone weighing enrollment is to read the stated phase and purpose of a trial first, since each stage answers a different question.
