Clinical trials take so long because nearly every stage, from paperwork to patient enrollment to data collection, involves built-in delays that compound over time. Only about 1 in 5 trials finishes within its planned timeframe. The rest run over by a median of roughly 12 months, and many stretch years beyond their original end dates. Understanding where that time goes helps explain why a process that seems straightforward on paper becomes a decade-long journey in practice.
Most Trials Can’t Find Enough Volunteers
Recruitment is the single biggest bottleneck. Among trials that do eventually finish, more than 70% experience significant delays, and difficulty enrolling participants is consistently the most common reason trials stall or get discontinued entirely. When trials fall short of their enrollment targets, the median shortfall is about 31% of the planned sample size. That means a trial designed to enroll 1,000 people might end up 300 patients short, forcing researchers to extend recruitment periods by months or years.
Why is it so hard to find volunteers? Most people never hear about trials relevant to them. Those who do may not meet the eligibility criteria, which have grown increasingly strict over time. Others are deterred by the time commitment: frequent clinic visits, blood draws, imaging scans, and detailed symptom tracking. For trials requiring specific patient populations (a rare cancer subtype, for example), the pool of eligible participants may be tiny and scattered across the country, making enrollment a slow trickle rather than a steady flow.
Regulatory Reviews Add Months Before a Trial Starts
Before a single patient can be enrolled, the trial protocol has to clear multiple layers of review. Every institution that participates in a trial has an ethics board (called an Institutional Review Board, or IRB) that must approve the study design, consent forms, and safety monitoring plan. Across the United States, median IRB review times run about 63 days. But that’s just the middle of the range. At some institutions, full board reviews take a median of 131 days, and individual reviews can stretch past 500 days when revisions are requested.
These timelines reflect the complete process from initial submission to final approval memo, including back-and-forth revisions. A national working group has suggested that IRBs should be able to review 90% of protocols within 60 days, but many institutions fall well short of that benchmark. And IRB approval is just one piece. Researchers also need regulatory clearance from the FDA for new drugs, plus approvals from each hospital’s legal and finance departments to finalize contracts. Each of those steps has its own timeline and its own potential for delay.
Contract Negotiations Slow Down Every Site
A large trial might involve dozens or even hundreds of hospitals and clinics across multiple countries. Each site needs a signed contract spelling out budgets, liability, intellectual property rights, and data-sharing terms. These negotiations happen between the trial sponsor and each institution’s legal team, and they can drag on for months. Budget disagreements are common: hospitals want to be fairly compensated for staff time and overhead, while sponsors want to control costs across a sprawling network of sites.
Site activation, the process of getting a location fully set up and ready to enroll patients, is widely recognized as a major bottleneck in drug development. A site can’t start recruiting until contracts are signed, supplies are shipped, staff are trained, and local regulatory approvals are in hand. If even a few key sites are delayed, the entire trial’s enrollment timeline suffers.
Trial Designs Have Gotten More Complex
Clinical trials today are significantly more complicated than they were a decade ago. A machine learning analysis of more than 16,000 trials found that average complexity scores increased by more than 10 percentage points over the past ten years. This rise is driven by several factors: more endpoints being measured per trial, more eligibility criteria screening patients in or out, more treatment arms comparing different doses or combinations, and more participating sites spread across more countries.
Cardiovascular trials, for instance, saw large jumps in complexity as wearable devices became widely adopted, enabling researchers to track more health metrics but also adding layers of data collection and analysis. Some industry observers now describe early-phase trials as having the complexity that used to be reserved for late-stage trials. More complexity means more data to collect per patient, more visits, more monitoring, and ultimately more time.
Each Phase Takes Years on Its Own
Drug development moves through distinct phases, and each one takes time for a reason. Phase 1 trials test safety in a small group and typically run one to two years. Phase 2 trials test whether the treatment actually works and enroll more patients over two to three years. Phase 3 trials are the largest, often involving thousands of participants across many sites, and can take three to four years or more. After that, the company compiles all the data and submits it to regulators for review, which adds another year or more.
Critically, these phases traditionally run in sequence. You don’t start Phase 2 until Phase 1 data proves the drug is safe enough to continue. You don’t start Phase 3 until Phase 2 shows promising effectiveness. Each transition point requires data analysis, protocol revisions, new regulatory submissions, and fresh recruitment. The cumulative effect is that bringing a single drug from first-in-human testing to market approval often takes eight to twelve years.
High Failure Rates Mean Starting Over
Not every delay is about logistics. Many trials simply fail. About 66% of drugs that enter Phase 1 advance to Phase 2. From there, only 58% make it to Phase 3. And of those that reach Phase 3, roughly 59% ultimately win approval. That means for every 100 drugs entering Phase 1, only about 23 will eventually reach patients. Each failure sends researchers back to earlier stages or to entirely new drug candidates, resetting the clock.
These failures aren’t wasted time in a scientific sense, since they reveal what doesn’t work, but they explain why the overall timeline from laboratory discovery to pharmacy shelf is so long. The average out-of-pocket cost to develop a single drug is estimated at around $173 million, with clinical trials alone accounting for about 68% of that spending. When you factor in the cost of all the drugs that failed along the way, the total investment per successful drug climbs to roughly $879 million. This financial reality means sponsors are cautious about moving forward, adding extra checkpoints and analyses that extend timelines but reduce the risk of an expensive late-stage failure.
Why COVID-19 Vaccines Were Different
The speed of COVID-19 vaccine development, roughly 11 months from sequence to emergency authorization, makes every other trial timeline look glacial. But it didn’t happen by skipping steps. It happened by running them in parallel and absorbing enormous financial risk. Normally, you wait for Phase 1 safety data before designing Phase 2. During the pandemic, companies ran Phase 1 and animal studies simultaneously. They began manufacturing at commercial scale before knowing whether their vaccine would even work, a gamble that would have wasted billions if the trials had failed.
Governments underwrote much of that financial risk through advance purchase agreements and direct funding, removing the biggest reason companies normally proceed cautiously. Recruitment was also unusually fast because a global pandemic meant millions of motivated volunteers and high rates of infection, which made it quicker to measure whether vaccines actually prevented disease. This combination of parallel execution, massive funding, and an urgent public willing to participate compressed what normally takes a decade into less than a year. It was not a model that scales easily to a rare liver disease or a slow-progressing neurological condition.
Remote Trials Could Shorten Timelines
One structural change gaining traction is the decentralized trial, where participants engage from home using telemedicine visits, mobile health apps, and remote monitoring devices instead of traveling to a hospital for every check-in. Traditional trials face geographic barriers that limit who can realistically participate. If you live three hours from the nearest trial site, weekly visits may be impossible. Decentralized models remove that obstacle, potentially speeding enrollment and reducing the dropout rates that force trials to extend their timelines.
These approaches are still maturing. Challenges remain around data security, ensuring remote measurements are as reliable as in-clinic ones, and reaching populations that lack broadband access or comfort with digital tools. But the core promise is real: by making participation less burdensome, decentralized trials could chip away at the recruitment delays that account for so much of the overall timeline.