Clinical trials move through up to five distinct phases, starting with tiny doses in a handful of people and ending with long-term monitoring after a drug reaches the market. Each phase answers a different question: Is this safe? Does it work? Is it better than what already exists? The entire process, from first human dose to approval, takes roughly a decade on average, and only about 1 in 5 drugs that enter Phase 1 testing ever reach patients.
Phase 0: Microdosing
Phase 0 is an optional, exploratory step that not all drugs go through. The goal isn’t to test whether a drug works or is safe in the traditional sense. Instead, researchers give a tiny amount of a compound to a small group of volunteers to see how the body absorbs, distributes, and breaks it down. These doses are less than 1/100th of the amount expected to have any therapeutic effect, capped at under 100 micrograms. At quantities that small, the drug produces no pharmacological activity and is unlikely to cause side effects.
The value of Phase 0 is speed and cost savings. By learning early how a drug behaves in the human body, researchers can weed out compounds with poor pharmacokinetic profiles before investing in the expensive safety testing of Phase 1.
Phase 1: Safety and Dosing
Phase 1 is where real safety testing begins. The primary goal is to find the right dose to carry forward into later trials while cataloging side effects. These studies typically enroll small groups and use a dose-escalation approach: the first few volunteers receive a low dose considered safe based on animal studies, and subsequent groups receive progressively higher doses.
The most common design, called the 3+3 method, treats patients in cohorts of three. If none of the three experience a serious side effect, the next three receive a higher dose. If one does, three more are treated at the same level before deciding whether to escalate. Dose increases follow a pattern where early jumps are large (sometimes doubling) and later jumps get smaller, rising by 30 to 40 percent at a time. Escalation stops when roughly a third or more of patients in a cohort experience a dose-limiting toxicity.
About 75% of drugs that enter Phase 1 successfully move on to Phase 2. The median duration of this phase is roughly 1.5 to 2 years.
Phase 2: Does the Drug Actually Work?
Phase 2 trials shift the focus from safety to efficacy. Researchers are trying to answer a straightforward question: does this treatment show enough promise to justify the massive investment of a Phase 3 trial? These studies typically enroll a few dozen to a few hundred patients who have the disease or condition the drug is designed to treat.
Many Phase 2 trials use a two-stage design. A relatively small group, often fewer than 30 patients, receives the drug first. Researchers pause to analyze results at the end of this stage. If the drug appears ineffective, the trial stops early, saving time and sparing additional patients from an unproductive treatment. If results look promising, enrollment continues into the second stage.
This is the phase where most drugs fail. Only about 50% of drugs that enter Phase 2 advance to Phase 3, making it the biggest bottleneck in the entire development pipeline. The median duration is roughly 2.5 to 3 years.
Phase 3: Large-Scale Proof
Phase 3 trials are the pivotal studies that regulators rely on when deciding whether to approve a drug. They enroll 300 to 3,000 volunteers with the target disease or condition and typically run for one to four years. The purpose is to confirm that the drug provides a real treatment benefit in a broader population and to detect adverse reactions that might not have appeared in smaller studies.
Unlike earlier phases, Phase 3 trials almost always include a comparison group. Some participants receive the experimental drug while others receive a placebo or the current standard treatment. This controlled design is what allows researchers to distinguish genuine drug effects from natural improvement or the placebo effect. About 59% of drugs that complete Phase 3 move on to the regulatory submission stage, and 88% of those submissions ultimately receive approval.
Phase 3 is also by far the most expensive stage of development. Median costs for a single Phase 3 program have been estimated between $99 million and $150 million, depending on whether the drug targets a rare disease or a common condition, with averages reaching $163 million across all trial types.
Phase 4: After Approval
Phase 4 studies begin after a drug has been approved and is available to patients. Regulatory agencies can require these studies as a condition of approval, and they serve several purposes that earlier trials simply can’t address. Some focus on a specific side effect that was flagged during earlier testing but needs longer observation. Others conduct broad surveillance of overall safety in the general population, where patients are more diverse and often taking other medications. Still others measure whether the drug’s benefits hold up over years rather than the months tracked in Phase 3.
These post-marketing studies are important because clinical trials before approval, even large Phase 3 trials, involve at most a few thousand carefully selected participants. Rare side effects that occur in 1 out of every 10,000 or 100,000 people may only surface once millions are taking the drug in real-world conditions.
Overall Odds From Start to Finish
Multiplying the success rates across all phases gives an overall likelihood of approval of about 19% for a drug entering Phase 1. That means roughly 4 out of 5 experimental drugs fail somewhere along the way. Phase 2 is the steepest drop-off, which makes sense: it’s the first time a drug is tested for actual effectiveness rather than just safety. Many compounds that are safe enough simply don’t work well enough.
The total timeline from Phase 1 through approval adds up to roughly 6 to 10 years of clinical testing alone, not counting the years of preclinical research that come before the first human dose.
Adaptive Trials and Flexible Designs
The traditional model of rigid, sequential phases is increasingly being supplemented by adaptive trial designs. In a conventional trial, the treatment groups, dosing, and allocation are locked in at the start and don’t change regardless of what happens during the study. Adaptive trials build in pre-planned checkpoints where researchers analyze incoming data and make adjustments. They might drop a treatment arm that clearly isn’t working, shift more participants toward a dose that appears most effective, or modify enrollment criteria.
This flexibility makes trials more efficient. Fewer patients end up receiving ineffective treatments, and promising drugs can sometimes move through the process faster. Adaptive designs are especially useful when evaluating multiple interventions at once, because underperforming options can be eliminated mid-trial rather than after the entire study is complete.
Diversity Requirements in Modern Trials
A drug that works well in one demographic group doesn’t always work the same way in another, which is why participant diversity has become a regulatory priority. Under legislation passed in 2023, sponsors running Phase 3 trials are now required to submit a Diversity Action Plan to the FDA. These plans must include specific enrollment goals broken down by race, ethnicity, sex, and age group, along with an explanation of how the sponsor intends to meet those goals. The requirement also applies to certain medical device studies. The FDA published draft guidance in June 2024 laying out how these plans should be developed, and the requirement takes effect 180 days after the final guidance is published.