Clinical trials are research studies that test medical treatments, drugs, or devices in human volunteers to find out whether they’re safe and effective. They are the required step between laboratory research and any new treatment reaching your doctor’s office. Every prescription drug, vaccine, and medical device available today went through clinical trials before approval, and roughly 14% of all drug development programs that enter testing eventually make it to the finish line.
How Clinical Trials Are Structured
Most clinical trials follow a carefully controlled design meant to eliminate bias and produce reliable results. The gold standard is the randomized, double-blind, placebo-controlled trial. In this setup, participants are randomly assigned to receive either the experimental treatment or an inactive placebo, and neither the participants nor the researchers know who is getting which. This matters because expectations alone can influence how people feel and report symptoms. If a patient knows they’re getting the real drug, or a doctor knows which group a patient belongs to, it can subtly skew the results.
Randomization also prevents a problem called selection bias, where doctors might unconsciously assign healthier patients to one group. By letting chance decide, the two groups end up roughly equal in age, health status, and other factors that could affect the outcome. Some trials use a “triple-blind” approach, where even the people analyzing the data don’t know which group is which until the study is complete.
The Four Phases of Testing
New drugs move through four distinct phases, each with a different goal and a progressively larger group of participants. The process is designed so that a treatment must clear one safety and effectiveness bar before advancing to the next.
Phase 1 is the first time a treatment is tested in people. It typically involves 20 to 100 volunteers and focuses on safety: what dose the body can tolerate, how the drug is absorbed and processed, and what side effects appear. About 70% of drugs pass this stage.
Phase 2 expands to up to several hundred people who have the disease or condition being studied. The goal shifts to whether the treatment actually works, while continuing to monitor side effects. These studies can last several months to two years, and only about 33% of drugs advance.
Phase 3 is the large-scale test, enrolling 300 to 3,000 volunteers with the condition. Researchers compare the new treatment against existing standard treatments or a placebo over one to four years, looking for clear evidence of effectiveness and tracking adverse reactions across a much larger population. Only 25 to 30% of drugs make it through. Before a company can apply for approval, it generally needs strong data from two large, controlled Phase 3 trials.
Phase 4 happens after a drug is already approved and on the market. Pre-approval trials can only study a few thousand people under tightly controlled conditions, with strict rules about who qualifies. Phase 4 studies track how the drug performs in the real world, where patients may be older, pregnant, taking multiple medications, or dealing with other health problems. These studies can reveal rare side effects that didn’t appear in smaller trials, identify new uses for the drug, or flag safety concerns that lead to label changes. In that sense, Phase 4 never truly ends.
Who Can Participate
Every trial has a list of eligibility requirements, divided into inclusion criteria (characteristics you must have) and exclusion criteria (characteristics that disqualify you). Inclusion criteria typically specify the disease or condition being studied and often require a certain severity level. You might need to be within a particular age range or have a specific subtype of a disease.
Exclusion criteria exist to protect participants and keep results interpretable. People with certain chronic conditions, impaired kidney or liver function, or those taking medications that could interact with the study drug are commonly excluded. In one analysis, more than 60% of drug trials excluded people based on liver function test results, and 58% set cutoffs based on kidney function. About 27% of trials for diseases common in older adults excluded participants based on age alone. Pregnant and lactating women have historically been excluded due to uncertainty about risks to the fetus or newborn.
These restrictions mean that trial participants often don’t perfectly represent the broader population who will eventually use the drug, which is one reason Phase 4 real-world monitoring is so important.
Protections for Participants
Before you can join a clinical trial, you go through a process called informed consent. This isn’t just signing a form. Federal regulations require researchers to explain the study’s purpose, any foreseeable risks or discomforts, potential benefits, and what alternative treatments already exist. You must be told how your medical information will be kept confidential, what compensation is available if you’re injured during the study, and who to contact with questions. Participation is always voluntary, and you can withdraw at any time without penalty.
Beyond informed consent, every trial must be reviewed and approved by an independent ethics committee (called an Institutional Review Board in the U.S.) before it can begin. The FDA also reviews trial data at multiple stages. After all phases are complete, FDA review teams examine every piece of submitted data before deciding whether to approve a treatment. Even after approval, the agency continues monitoring safety for as long as the product is on the market.
Benefits and Risks of Participating
People join clinical trials for different reasons. Some hope to access a promising new treatment before it’s widely available, particularly for serious conditions where current options are limited. Others want to contribute to research that could help future patients. Participants often gain a deeper understanding of their condition and receive closer medical monitoring than they would in routine care.
The risks are real, though. An experimental treatment may cause side effects ranging from mild to serious, and there’s no guarantee it will work or be better than what’s already available. If you’re in a placebo-controlled trial, you might be assigned to the group that doesn’t receive the active treatment at all. Participation can also be time-consuming, requiring extra medical appointments, additional tests, complex medication schedules, or hospital stays. And while researchers take precautions to protect your privacy, other people involved in the study (such as sponsors or safety monitors) may have access to your medical information.
The Scale of Clinical Research Today
Clinical trials are a massive global enterprise. The United States has registered the most trials since systematic tracking began in 1999, with over 197,000 as of mid-2025. China follows with roughly 164,000 and India with about 94,000. Of all registered trials with an identified phase, Phase 2 trials make up the largest share at nearly 116,000, reflecting the high volume of treatments being tested for initial effectiveness. Most trials (74%) include both men and women, and 72% focus on adults, while only 12% include children.
The overall odds of success remain low. A large analysis of industry drug development programs found that only about 13.8% of drugs that enter Phase 1 testing eventually reach approval. The steepest drop-off happens between Phase 2 and Phase 3, where two-thirds of drugs fail after showing early promise. This high attrition rate is one reason drug development is so expensive and time-consuming, but it’s also what makes the system work: treatments that reach patients have been tested rigorously enough that their benefits are well-established and their risks are known.