How Long Do Clinical Trials Take? Understanding Clinical Trial Timelines

The average length of clinical trials can vary from study to study. If you’re considering clinical trials, learn everything you need to know here.

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Reviewed by Susan Egbert, PharmD

Published 20 December 2023

Clinical trials are essential in the world of medical research. Without them, we wouldn’t have crucial insights into the safety and efficacy of new treatments, medications, and interventions. But how long do clinical trials take?

The length of a clinical trial will depend on several different factors and can be anything from a few days to several decades. Understanding clinical trial lengths is paramount for both researchers and participants alike. This knowledge enables everyone involved to make informed decisions and understand the impact of their involvement.

In this guide, we’ll explore the average length of clinical trials and factors that can impact clinical trial timelines.

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How Long Do Clinical Trials Take for Drugs?

The average clinical trial duration for drugs is between six and ten years. The duration will depend on several considerations, including:

  • The specific drug
  • The complexity of the disease
  • The condition being studied
  • Other factors that could occur during the trial

The overall duration will also depend on the time it takes for preclinical research, regulatory approval, and the clinical trial phases. From initial discovery to full approval, you can expect a new drug to be on a journey for approximately 10 years, with clinical trials taking up around seven years of this.

How Long Do Clinical Trial Phases Take?

There are multiple phases in a single clinical trial, with each phase fulfilling a different purpose. Although each phase has the shared purpose of testing a treatment until it is deemed safe and effective.

Not all clinical trials will have the same amount of phases, it all depends on the study and treatment being tested. The main phases are one to three, with some trials having an earlier stage zero trial and others having a phase four trial after a drug has been licensed.

How long do phase 0 clinical trials take?

Phase 0 trials, or exploratory trials, usually have the shortest duration of all phases. The average duration is typically a few days to a few weeks and only a handful of participants are required to take part.

This is because phase 0 is designed to offer proof-of-concept rather than identification of a dose. Not all clinical trials will include this phase in their study.

How long does a phase 1 clinical trial last?

As the initial phase of a clinical trial, the average length of phase 1 clinical trial is around two years. However, it could last as little as six months or over five years for drugs that require a longer development period.

Phase 1 trials usually go ahead with between 20 and 100 participants, typically involving healthy volunteers or patients. On average, phase 1 trials take less time to complete than later phases of a trial.

How long do phase 2 clinical trials take?

Phase 2 trials usually last from several months to a few years but vary depending on the study requirements. Around 70% of drugs progress to the second phase after the initial safety assessment has been completed in phase 1.

This phase is designed to measure safety in a larger group of volunteers (typically several hundred). It also measures the effectiveness of the treatment and any side effects.

How long do phase 3 clinical trials take?

The average length of phase 3 clinical trials is between two and four years. They usually last several years and aim to compare the new treatment with the best currently available treatment.

Phase 3 trials usually involve more participants than phases 1 and 2 because the success rate difference could be very small. You can expect anywhere between 300 and 3,000 participants in phase 3.

How long do phase 4 clinical trials take?

Phase 4 in a clinical trial can last for several years or continue indefinitely in some cases. This phase is optional in many studies and aims to assess the effects of the new treatment after approval.

It will typically involve looking for side effects that were not picked up earlier and studying how well the treatment works over time.

How Long Do Clinical Trials Usually Last for Participants?

While a clinical trial can take several years, a participant's involvement in it could be significantly less. This is because they will likely only be involved for one or two phases of the trial, which can last anywhere from a few days to several years.

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Want to learn more about participating in clinical trials? Read our comprehensive guide.

Everything You Need To Know About Participating In Clinical Trials

Factors That Could Affect the Duration of a Clinical Trial

We know that the average duration of a clinical trial can vary from study to study. But several factors can influence this.

The number of patients needed and recruitment challenges

Also known as the ‘enrollment rate’, the time it takes to recruit the required number of participants for a trial can significantly impact the trial duration. A slow enrollment could extend the study timeline and delay the next phases.

Researchers might also find it difficult to find the right type of participants, thus delaying the start of the trial. Strict eligibility criteria, including age, medical history, or pre-existing conditions can limit the pool of eligible participants.

The type of clinical trial and study design

There are several different ways a study can be designed and run. The complexity of the study could include factors like several treatment arms, different randomization procedures, or complex data retrieval methods. Each of these can influence the time required for the trial.

There may also be logistical challenges with the study design. For example, site setup, training, and specific protocols could prolong the trial duration.

The type and complexity of the condition being studied

Some conditions are more common than others, which means it’s often easier to find participants. For example, clinical trials for rare cancers often take longer than clinical trials for more common cancers. This is because there is a larger pool of people to select from.

The type of treatment being administered

Clinical trials testing new methods of administration often need longer to set up and run, which could impact the trial duration. The research teams may also need extra training and often require specialist equipment which could take longer to arrive than readily available equipment.

Potential issues with the new treatment

Until the clinical trial begins, researchers might be unaware of potential problems with the new treatment. This could impact the duration of the trial twofold. Some trials may have to be stopped completely, therefore shortening the duration. Others will need to be extended as researchers work to resolve the problem.

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Learn more about the potential risks, benefits and safety considerations in our guide.

Clinical Trial Benefits, Risks & Safety Considerations

Unexpected adverse events and safety concerns

Researchers often come up against factors out of their control. Unforeseen adverse events during the trial can disrupt the study, requiring additional investigations, adjustments to the treatment protocol, or even suspension of the trial.

Safety is paramount in clinical studies. However, this often means that rigorous safety monitoring protocols identify potential safety concerns that require further investigation or trial modification, resulting in delays.

Difficulties in patient retention

While participants would have signed up and agreed to take part for the duration of the study, this doesn’t always happen. If participants leave the trial part way through, this can impact data collection and delay the entire study if replacement participants are required.

The follow-up period

A participant might only be an active part of the trial for a short time, but they could be required for follow-ups for years after their involvement. Research teams look at how a treatment has impacted a participant in the long term, ensuring the treatment works as it should over a long period.

Regulatory and ethical considerations

Clinical trials must adhere to strict regulatory guidelines and ethical principles. Unfortunately, this can be complex for some studies and potentially lead to delays in the trial commencement and data analysis.

Likewise, obtaining regulatory approvals like Investigational New Drug (IND) applications or ethics committee clearance can be time-consuming and lead to delays.

Budgetary constraints and resource limitations

There is a substantial cost involved in clinical trials, and problems with the budget or resources can be hugely damaging for trial durations. Resource availability can impact the beginning of the trial and affect the overall timeline. For example, some studies might require specialized equipment that might not be readily available.

Studies require funding for things like site management, data collection and analysis, participant compensation, and monitoring. All of these costs add up, and if the funding is delayed or under budget, the trial will inevitably be delayed or suspended.


Depending on the trial, the duration really can vary. However, speaking to the responsible research team can help you further understand the specifics of your study and how long you are expected to be an active participant.

Whether you’re considering taking part in a trial or you just want to know a little more about them, our guide should give you plenty to think about.

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