A clinical trial is a structured research study that tests whether a new medical treatment is safe and effective in people. The process moves through a series of phases, each with more participants and stricter standards, before a drug or therapy can reach the market. On average, only about 14% of drugs that enter the first phase of testing ever win FDA approval, and the entire journey from early testing to approval typically takes a decade or longer.
The Four Phases of Testing
Clinical trials follow a phased structure, with each stage answering a different question. A treatment can only advance to the next phase if it clears the previous one.
Phase 1 is the first time a treatment is tested in humans. It involves 20 to 100 people, often healthy volunteers, and lasts several months. The goal is narrow: find a safe dosage range and identify obvious side effects. Researchers aren’t trying to prove the drug works yet. They’re making sure it doesn’t cause unacceptable harm.
Phase 2 widens the pool to up to several hundred people who actually have the disease or condition being targeted. This phase, which can last from several months to two years, is where researchers get a first real look at whether the treatment does what it’s supposed to do. Side effects are tracked more carefully, and dosing is refined.
Phase 3 is the big one. Between 300 and 3,000 volunteers with the condition participate over one to four years. This phase generates the large-scale evidence regulators need to decide whether to approve a treatment. It confirms effectiveness, compares the new treatment against existing options, and monitors for rarer adverse reactions that smaller studies might miss. Most drugs that fail in clinical development fail here.
Phase 4 happens after a drug has already been approved and is on the market. These studies involve several thousand people and track long-term safety and effectiveness in the broader population. Sometimes Phase 4 data reveals side effects that didn’t show up during earlier, shorter trials.
Who Can Join a Trial
Every trial has a specific set of eligibility rules. Inclusion criteria define who the study is looking for: people of a certain age range, with a confirmed diagnosis, living in a particular area, or at a specific stage of disease. Exclusion criteria filter out people who technically qualify but whose participation could compromise the data or put them at higher risk. Someone with an additional health condition that could muddy the results, for instance, or someone unlikely to be able to attend follow-up appointments, might be excluded.
These criteria aren’t arbitrary. A lung disease trial, for example, might include adults over 40 with a stable COPD diagnosis who are current or former smokers, but exclude people with sleep apnea or other chronic respiratory conditions. The tighter the criteria, the cleaner the data, though overly narrow rules can make results harder to generalize to the real world.
Randomization and Blinding
The most rigorous trials use a design called randomized, double-blind, placebo-controlled. Here’s what each piece means in practice.
Randomization means participants are assigned to groups by chance, not by a doctor’s judgment. One group receives the experimental treatment, and the other receives either a placebo (an inactive substitute) or the current standard treatment. This prevents the groups from being stacked in ways that skew results.
Blinding controls for bias. In a single-blind study, participants don’t know which group they’re in. In a double-blind study, neither the participants nor the researchers interacting with them know who is getting the real treatment. This matters because expectations can influence how people report symptoms, and even how doctors assess outcomes. If a researcher believes a patient is on the active drug, they might unconsciously interpret ambiguous signs more favorably.
When Placebos Are and Aren’t Used
Placebos are essential for measuring whether a treatment’s effects are real or driven by the psychology of receiving care. But giving someone a sugar pill when a proven treatment already exists raises serious ethical questions.
International guidelines, laid out in the Declaration of Helsinki, draw a clear line. Placebos are acceptable when no proven treatment exists for the condition. When an effective treatment is already available, a new drug is generally tested against that existing standard rather than a placebo. Placebos can still be used alongside an existing treatment if there’s a strong scientific reason, but only when withholding proven therapy won’t expose participants to serious or irreversible harm.
Informed Consent
Before joining any trial, you go through an informed consent process. This isn’t just signing a form. Federal regulations require that you receive specific information: what the study involves, how long your participation will last, what risks and discomforts are foreseeable, what benefits you might expect, and what alternative treatments are available to you outside the trial. You must also be told how your personal data will be kept confidential, whether compensation or medical treatment is available if something goes wrong, and who to contact with questions.
One point the regulations make explicit: participation is voluntary. You can refuse to join without any penalty, and you can withdraw at any time without losing access to care or benefits you’re otherwise entitled to.
Safety Monitoring During the Trial
Two layers of oversight protect participants throughout a trial.
The first is an Institutional Review Board, or IRB. This is an independent committee that reviews a study’s design before it begins and at least once a year while it’s running. The IRB evaluates whether risks to participants are minimized, whether those risks are reasonable given the potential knowledge gained, and whether the consent process is adequate. An IRB has the authority to approve, require changes to, or shut down a study entirely. It can suspend research immediately if it identifies serious harm or noncompliance with the rules.
The second layer is a Data Safety Monitoring Board, or DSMB, which reviews the actual data as the trial progresses. The DSMB looks at safety data, side effects, and preliminary efficacy results. It can recommend stopping a trial early for several reasons: the treatment is causing too much harm, it’s clearly not working, or, on the other end, it’s working so well that it would be unethical to keep the control group on a placebo. A DSMB also watches for practical problems like slow enrollment, poor adherence to the study protocol, or high dropout rates that could make the trial’s conclusions meaningless.
What Happens After the Trial
If a treatment survives all three phases, the company compiles the data into a formal application for regulatory review. The FDA (in the United States) or equivalent agencies in other countries examine the evidence and decide whether to approve it. This review process itself can take months to over a year.
The financial stakes are enormous. One analysis by the U.S. Department of Health and Human Services estimated the average out-of-pocket cost to develop a single drug at about $173 million. When you factor in the cost of all the drugs that failed along the way, plus the opportunity cost of capital tied up for years, the figure rises to roughly $879 million per approved drug. That’s why only large pharmaceutical companies and well-funded biotech firms can typically afford to run late-stage trials.
How to Find a Trial
If you’re interested in joining a clinical trial, ClinicalTrials.gov is the largest public database of studies worldwide. You can search by condition, location, and trial status to find studies that are actively recruiting participants. For cancer specifically, the National Cancer Institute maintains its own searchable database of NCI-funded trials and offers a helpline (1-800-422-6237) and live chat for people who need guidance navigating their options.
Your doctor can also help identify trials relevant to your condition, and many academic medical centers maintain lists of studies they’re currently running. Trial listings include contact information for the research team, the eligibility criteria, and what participation will involve, so you can evaluate whether a study is a realistic fit before committing.