Clinical trials are vital for testing new drugs, treatments, and medical interventions. They are used to ensure the safety, efficacy, and of course, benefits of new treatments to patients before they can be introduced to the general public.
These trials are typically divided into four or five key phases. In this guide, we’re going to break these down into more detail and take a look at what happens at each stage.
Phase one: Exploratory trials
This stage, sometimes referred to as Phase Zero, is the very first stage of the process. The purpose of this early phase is small-scale trials that gather preliminary data on how a drug, treatment, or intervention interacts with the body.
Typically, fewer than 15 participants are involved at this point, keeping these exploratory trials low-key.
The focus is to determine how the drug or treatment behaves and the impact it has on the body. For example, when testing a new drug, the idea is to discover how it is absorbed, distributed, and broken down. At this stage, scientists and medical professionals may not focus too much on the therapeutic effects.
This phase is usually very short, often just a few days in total. This can provide early insights into the dosage, techniques, or amount of treatment required for future trials.
Phase two: Safety and dosage testing
The next phase of the trial focuses on the safety of a new drug, intervention, or treatment, determining the safest dose, exposure, or techniques. This is designed to identify any side effects and evaluate how the body metabolizes drugs or behaves under certain treatments.
Phase two typically lasts for a couple of months and has around 20 to 100 healthy participants or sometimes people with the targeted disease (depending on what is being tested).
The intended outcome is to establish the safest dose or treatment, ensuring it produces the least side effects. This is known as the ‘maximum tolerated dose’.
Phase three: Efficacy and side effects
Once the safest and most effective dose has been determined, it’s time to evaluate the effectiveness of said drug, treatment, or intervention, while continuing to monitor its safety. The number of participants will once again increase, this time to between 100 and 300 patients with the targeted condition.
The key focus is to test whether the drug or treatments work (their efficacy), identify the optimal dosage or length of treatment and continue to monitor any short-term side effects.
This can take anywhere between a couple of months all the way up to two years and is an integral part of the trial as it provides more in-depth data on whether it will work well enough to proceed to larger-scale testing.
Phase four: Larger-scale efficacy testing
The next stage is vital to confirm the effectiveness of a drug, treatment, or intervention by testing efficacy on a much larger scale. Up to ten times the original group, in fact, with anywhere between 300 and 3,000 patients (sometimes more). These participants might even be from multiple locations around the world.
During phase four, it’s also important to monitor side effects and compare the new drug or treatment to standard therapies and those already on the market.
This is because testing new drugs or treatments on a larger population provides definitive evidence of their safety and efficacy. At this stage, it’s also possible to identify any of the rarer or longer-term side effects as this phase typically takes place over one to four years.
If successful, the data gathered during this stage can be submitted to regulatory authorities for approval of the new drug, treatment, or intervention.
Phase five: Post-market surveillance
Lastly, phase five takes place after a new medication or treatment has been approved and is now available on the market. The purpose of this stage is to monitor the long-term effectiveness and to identify any side effects that may have gone undetected during the initial trial.
As a result, this phase uses thousands of patients in real-world clinical settings to get more detailed data from a much wider scope.
The key focus here is to observe any long-term risks and benefits and to identify rare side effects that suddenly become apparent, as well as evaluate the new treatment's performance when used on the general population.
This phase is ongoing and should continue for as long as the drug or treatment is in use.
The data gathered may lead to additional regulatory actions. For example, the changing of labels or possibly even withdrawing the drug from the market if necessary.
A summary of the five phases
Phase one: Exploratory trials on a small scale that gather preliminary data on how a drug, treatment or intervention interacts with the body
Phase two: Safety and dosage testing to determine the safest dose, exposure or techniques
Phase three: Testing whether the drug or treatments work and monitoring for any short-term side effects
Phase four: Bigger efficacy testing and monitoring for adverse reactions on a larger scale with more participants
Phase five: Post-market surveillance to monitor the long-term effectiveness and any side effects of drugs or treatments once they are on the market
The benefits of conducting clinical trials in different phases
Each phase builds on the last, progressively testing new drugs and treatments in more people and under more rigorous conditions. There are several key benefits of conducting clinical trials in this way, and these include:
Increased safety - The risk of any new treatment is gradually assessed to protect participants
Ensuring efficacy - Similarly, efficacy is tested incrementally allowing scientists and medical professionals to make informed decisions about whether or not the clinical trial objectives will be met and therefore, whether or not to continue
Cost-effectiveness - Resources are allocated based on the success of earlier phases helping to save money and ensure the most cost-effective trials
Long-term monitoring and regulatory approval - The ongoing oversight of phase five ensures compliance and guidance, supporting the ongoing safety of users
Ethical considerations - Gradual testing in this way makes it easier to uphold ethical guidelines
Breaking clinical trials into phases ensures that treatments are evaluated carefully and thoroughly, balancing the need for innovation with patient safety.
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