Clinical trials have four main phases, numbered 1 through 4, that take a drug from first-in-human testing to long-term monitoring after it hits the market. Some sources count a fifth stage called Phase 0, which is a small exploratory study that sometimes happens before Phase 1. Each phase has a different goal, a different number of participants, and a different price tag. Understanding how they build on each other helps make sense of why drug development takes so long and costs so much.
Phase 0: The Optional Exploratory Step
Phase 0 isn’t required and doesn’t happen for every drug. The FDA describes it as an exploratory study conducted early in Phase 1 that involves “very limited human exposure” and has “no therapeutic or diagnostic intent.” In practice, that means a tiny dose of the drug, far too small to treat anything, is given to a handful of volunteers (usually fewer than 15) to see how the compound behaves in the human body. Dosing typically lasts no more than seven days.
The point is purely informational. Researchers want to confirm that the drug reaches the right tissues, gets absorbed the way animal studies predicted, and breaks down at the expected rate. If the results look promising, the drug moves into formal Phase 1 testing. If something unexpected shows up, the team can pivot early without having invested years and millions of dollars.
Phase 1: First Real Safety Test
Phase 1 is where a drug gets its first meaningful test in people. Typically 20 to 80 participants enroll, though some trials stretch to 100. Most volunteers are healthy adults, though for certain diseases like cancer, participants may already have the condition being studied.
The primary question is safety: how much of the drug can the body tolerate, and what side effects appear at increasing doses? Researchers start with a low dose based on animal data, then gradually increase it while closely monitoring for problems. Along the way, they also gather early clues about whether the drug is actually doing what it’s supposed to do in the body. The median cost of a Phase 1 trial is roughly $984,000, making it the least expensive of the three pre-approval phases. Most Phase 1 studies take several months to complete.
Phase 2: Does the Drug Actually Work?
If a drug clears Phase 1, Phase 2 shifts the focus from safety to effectiveness. Several hundred participants with the target disease or condition enroll, and researchers begin testing whether the drug produces a real therapeutic benefit. This is also where the optimal dose gets refined. Different groups of participants may receive different doses so researchers can figure out which one balances the best results with the fewest side effects.
Phase 2 trials are often where drugs fail. A treatment might be safe enough but simply not effective, or its side effects at therapeutic doses might be too severe. The median cost jumps to about $1.46 million, and these trials generally run one to two years. Because the participant pool is larger and the study design more complex, the data collected here shapes the entire strategy for Phase 3.
Phase 3: The Large-Scale Proof
Phase 3 is the final hurdle before a company can apply for FDA approval. These trials enroll hundreds to several thousand participants across multiple locations, sometimes spanning different countries. The goal is to confirm effectiveness in a large, diverse population and to catch rarer side effects that smaller studies might miss.
Most Phase 3 trials are randomized and controlled, meaning participants are randomly assigned to receive either the new drug or a comparison (a placebo or an existing treatment). Neither the participants nor the researchers typically know who is getting what, a design called double-blinding that reduces bias. These studies are expensive and slow: the median cost is about $2.5 million, and some large Phase 3 programs cost tens of millions. They often take three to four years to complete.
The data from Phase 3 forms the backbone of a New Drug Application, the formal request asking the FDA to approve the drug for sale. Once that application is submitted, the FDA has 60 days to decide whether to accept it for review. Reviewers then evaluate the safety and effectiveness data, inspect the manufacturing facilities, and approve the drug’s labeling before making a final decision.
Phase 4: Monitoring After Approval
Once a drug is on the market, Phase 4 begins. These post-marketing studies are sometimes required by the FDA as a condition of approval and continue tracking the drug in the real world. Pre-approval trials, even large Phase 3 studies, can only catch so much. Rare side effects that affect one in 10,000 people, interactions with other common medications, or problems that only emerge after years of use may not show up until millions of patients have taken the drug.
Phase 4 studies cover a wide range of questions. Some focus on a specific known side effect to measure how often it actually occurs. Others cast a broader net, conducting general surveillance for any adverse reactions. The FDA also requires studies in populations that may not have been well represented in earlier trials: children, infants, elderly patients, pregnant women, and specific racial or ethnic groups. Long-term efficacy studies fall into Phase 4 as well, confirming that a drug continues to work over years rather than just the weeks or months measured in Phase 3.
Phase 4 has no fixed endpoint. Monitoring continues for as long as a drug remains on the market, and serious safety findings can lead to new warnings, restricted use, or even withdrawal of the drug entirely.
How the Phases Add Up
From first-in-human dosing to FDA approval, the clinical trial process alone typically takes six to ten years. That doesn’t include the preclinical research (lab and animal studies) that comes before Phase 1, which can add another three to six years. The entire journey from discovery to pharmacy shelf often spans over a decade.
Cost escalates dramatically at each step. Phase 1 trials have a median cost just under $1 million, Phase 2 around $1.5 million, and Phase 3 about $2.5 million. But those medians can be misleading: complex diseases, large participant pools, and multi-site international trials push costs far higher. The total investment to bring a single drug to market, including the cost of all the drugs that failed along the way, is estimated in the billions.
Not every drug makes it through. The majority of compounds that enter Phase 1 never reach approval. Phase 2 is the most common point of failure, where promising safety profiles run into disappointing effectiveness results. Even drugs that complete Phase 3 can be rejected if the FDA determines the benefits don’t outweigh the risks. Each phase exists to filter out treatments that aren’t safe or effective enough, so that the drugs that do reach patients have been tested at progressively larger scales with increasingly rigorous standards.