A practical guide for patients and families trying to figure out where they fit in the clinical trial process.
If you or someone you care about is looking into clinical trials, the first thing you will notice is the numbering system. Phase 1, Phase 2, Phase 3. Most sources explain what each phase is, but very few help you figure out which one actually makes sense for your situation. That is what this guide is for.
Clinical trials are not a one-size-fits-all decision. The right phase for one patient could be completely wrong for another, depending on where they are in their treatment journey, how much risk they are comfortable with, and what their goals are. Below, we break down each phase from a patient’s perspective, so you can have a more grounded conversation with your medical team about what might be worth exploring.
A Quick Overview of Clinical Trial Phases
Before getting into the details, here is the big picture. Every new treatment goes through a sequence of testing stages before it can be approved by regulators like Health Canada or the FDA. Each phase has a different goal, a different number of participants, and a different level of risk.
| Phase 1 | Phase 2 | Phase 3 | |
|---|---|---|---|
| Primary goal | Safety and dosage | Does the treatment work? | Is it better than standard care? |
| Number of patients | 15 to 50 | 25 to 300 | 300 to 3,000+ |
| Typical duration | Several months | Months to 2 years | 1 to 4 years |
| Placebo used? | Rarely | Sometimes | Often (but usually alongside standard treatment) |
| Risk level | Highest | Moderate | Lower (more data exists) |
This table gives you the snapshot, but the real decision involves more context. Let us get into each phase.
Phase 1 Trials: First in Humans
Phase 1 is the first time a treatment is tested in people. Up until this point, it has only been studied in labs and, in many cases, in animals. The main question researchers are trying to answer is: Is this safe enough to keep testing?
In a Phase 1 trial, participants are usually split into small groups called cohorts. The first cohort gets a very low dose. If they tolerate it, the next group gets a slightly higher one. This continues until researchers find the highest dose that does not cause serious problems. You might hear this called a “dose-escalation” design.
Who typically joins Phase 1 trials?
For most drug types, Phase 1 trials enroll healthy volunteers. But in cancer research, it is different. Because the treatments themselves can be toxic, Phase 1 cancer trials usually enroll patients who have already been through standard treatments without success. These are often people with advanced or metastatic disease who are looking for options beyond conventional care.
As one oncologist at MD Anderson Cancer Center has noted, patients in Phase 1 trials are typically those who have tried standard chemotherapy, still have mild or manageable symptoms, and are not ready for hospice. They want to explore what else might be out there.
What are the real risks?
Phase 1 trials carry the most uncertainty. The treatment has never been given to people at these doses before, so unexpected side effects are a real possibility. Participants are monitored very closely, often with frequent blood draws, vital sign checks, and sometimes overnight hospital stays. That said, the monitoring itself can be a benefit. You are being watched more carefully than in almost any other medical setting.
Historically, about 47% of drugs tested in Phase 1 go on to Phase 2, which means over half do not make it past this stage. The overall chance of a drug going from Phase 1 all the way to market approval sits around 7 to 14%, depending on the disease area and the era of study.
Should you consider a Phase 1 trial?
You might consider a Phase 1 trial if you have run out of standard treatment options, if you have a good performance status (meaning your overall health is still reasonably strong), and if you understand that the primary benefit may be to future patients rather than yourself. That does not mean there is no chance of benefit. Some patients in Phase 1 trials do respond to the treatment, especially in the era of targeted therapies and immunotherapy. But going in with realistic expectations matters a lot.
If you are exploring early-stage trials, a tool like Horizon Trials’ AI matching platform can help surface Phase 1 opportunities that fit your specific diagnosis and medical history, saving weeks of manual searching.
Phase 2 Trials: Testing Whether It Works
Once a treatment passes Phase 1, the next question becomes: Does it actually do anything against the disease? That is what Phase 2 is designed to find out.
Phase 2 trials enroll a larger group of patients, typically between 25 and 300, all of whom have the specific condition the treatment targets. If Phase 1 was broad (testing across different cancer types, for example), Phase 2 narrows the focus. A Phase 2 trial for a lung cancer drug will only enroll lung cancer patients.
How Phase 2 trials are structured
Some Phase 2 trials are divided into two sub-phases. Phase 2a focuses on dosing, trying to confirm the ideal amount found in Phase 1. Phase 2b focuses on how well the drug actually works at that dose, measuring things like tumor shrinkage, disease stabilization, or biomarker changes. Not all trials split things this way, but it is common enough that you may see “Phase 2a” or “Phase 2b” when browsing trial listings.
Many Phase 2 trials are randomized. That means a computer, not a person, assigns you to a treatment group or a control group. Some use placebos, though in cancer trials this is less common. More often, the control group receives the current standard treatment, and the study group gets the new drug either alone or added on top of standard therapy.
Why Phase 2 is the toughest hurdle
This is where most drug candidates fail. Roughly two-thirds of treatments that enter Phase 2 do not make it to Phase 3. The reason is simple: Phase 1 only proves a drug is safe enough to keep testing. Phase 2 is where the drug has to show it actually works. Many drugs that looked promising in the lab just do not translate well to human patients.
For patients, this is important context. A Phase 2 trial gives you access to a treatment that has cleared basic safety checks but has not yet proven it works better than existing options. There is real hope here, but also real uncertainty.
Should you consider a Phase 2 trial?
Phase 2 trials may be a good fit if you have a clear diagnosis, your current treatment options are limited or not working well, and you are looking for something that has at least passed initial safety testing. The monitoring is still close, but typically less intensive than Phase 1. You will usually have fewer hospital visits and a more predictable schedule.
Patients who are curious about how clinical trial eligibility works should know that Phase 2 criteria tend to be more specific. You will usually need to have a confirmed diagnosis of the exact condition being studied, and you may need to meet certain lab value thresholds or have tried (and failed) certain prior treatments.
Phase 3 Trials: The Final Test Before Approval
Phase 3 is the last major step before a treatment can be submitted for regulatory approval. The central question is: Is this new treatment better than what we already have?
These trials are big. They typically enroll hundreds to thousands of patients across multiple hospitals and, frequently, multiple countries. The larger scale is not just for show. Researchers need large numbers to detect small but meaningful differences between treatments. If a new drug improves survival by a few months compared to the standard, you need a lot of patients to prove that the difference is real and not just random chance.
What to expect as a Phase 3 participant
Most Phase 3 trials are randomized and blinded. Randomized means you are assigned to a group by a computer. Blinded means that neither you nor your doctor knows which group you are in. This prevents bias from affecting the results.
In many Phase 3 cancer trials, every participant receives at least the standard-of-care treatment. The experimental group gets the new drug on top of that. So even if you are assigned to the control arm, you are still getting your normal treatment, not just a sugar pill. That said, some trials do use placebos alongside standard care, so always ask about the study design before signing the consent form.
Success rates at this stage
Phase 3 trials have a historically better track record than Phase 2, with roughly 55 to 60% of drugs that enter Phase 3 eventually moving on to regulatory submission. Once filed, about 90% of drugs receive approval. So if a treatment has made it to Phase 3, the odds are more in its favor, though they are far from guaranteed.
There is also an important safety net. If at any point during a Phase 3 trial it becomes clear that one group is doing much better than the other, the trial can be stopped early so that all participants can receive the better treatment.
Should you consider a Phase 3 trial?
Phase 3 is often the most comfortable entry point for patients who are new to clinical trials. The treatment being studied has already passed two rounds of human testing. There is more data on its safety and effectiveness. And the trial design usually ensures you get at least the standard of care, regardless of which arm you are assigned to.
If you are someone who wants to try something new but is not ready to accept the higher uncertainty of earlier phases, Phase 3 is often a reasonable middle ground. It also tends to be easier to find Phase 3 trials because they need far more participants.
What About Combined Phase Trials (Phase 1/2 or Phase 2/3)?
You will sometimes see trials listed as “Phase 1/2” or “Phase 2/3.” These combine elements from two phases to speed things up. A Phase 1/2 trial, for instance, might start by finding the right dose and then expand directly into testing whether the drug works, all within the same study.
This design is especially common for serious diseases like cancer, rare genetic conditions, and some infectious diseases where there is an urgent need for new treatments. For patients, a combined trial can mean faster access to a therapy and fewer delays between phases. But it also means the treatment has less of a track record at the time you join.
How to Decide Which Phase Is Right for You
There is no single right answer. The best phase for you depends on several factors that are unique to your situation.
Where are you in your treatment journey? If standard treatments are still working, a Phase 3 trial (which often includes standard care) might be the best fit. If you have exhausted standard options, Phase 1 or Phase 2 trials may open new doors.
How much uncertainty can you handle? Phase 1 has the least amount of human safety data. Phase 3 has the most. Be honest with yourself about your comfort level.
What are your goals? Some patients want to contribute to research and are comfortable with the possibility that the treatment may not help them personally. Others want the highest chance of direct benefit. Both motivations are valid, and they lead to different decisions.
What does your medical team recommend? Your oncologist or specialist knows your full medical picture. Not all doctors bring up clinical trials proactively, so do not be afraid to ask. If they suggest a specific trial, ask them which phase it is and why they think it fits your case.
How much time and energy can you invest? Phase 1 trials tend to require the most frequent visits. Phase 3 trials often follow a schedule closer to regular treatment. Think about how the trial logistics fit into your life.
Common Questions Patients Ask About Trial Phases
- Can I switch from one phase to another?
Not within the same study. Treatments move through phases, but patients do not. You participate in one phase of one trial. However, if a Phase 1 trial leads to a Phase 2 trial later on, you could potentially enroll in the next study if you meet the criteria.
- Do I have to pay to join a clinical trial?
The treatment itself is usually provided at no cost. But you may still face expenses for things like travel, parking, or additional medical tests. Some trials offer stipends or reimbursement. Always ask about the financial details before signing up.
- Will I definitely get the experimental treatment?
In Phase 1, almost always yes. In Phase 2 and 3, it depends on the study design. Randomized trials assign you to groups, and you might end up in the control arm receiving standard treatment or a placebo. In cancer trials specifically, placebos are rarely used alone. Most control arms still receive standard therapy.
What if I want to leave the trial?
You can withdraw at any time, for any reason. Participation is always voluntary. Your standard medical care will not be affected by your decision to leave a trial.
Finding Trials That Match Your Phase Preference
Searching for clinical trials on your own can be overwhelming. Government registries like ClinicalTrials.gov list thousands of studies, and the medical language in those listings can be hard to decode. Each listing tells you the phase, but figuring out whether you actually qualify takes more digging.
If you are in Canada, Horizon Trials was built to make this simpler. You create a medical profile, and the platform’s AI matching system scans active trials across Canada to find ones that fit your condition, history, and location. It saves significant time and puts relevant options in front of you that you might never have found on your own.
The Bottom Line
Understanding clinical trial phases is not just academic knowledge. It directly affects the kind of risk you are taking, the type of monitoring you will receive, and the likelihood that the treatment you are testing will eventually become an approved therapy. Phase 1 is about safety and discovery. Phase 2 is about whether the treatment works. Phase 3 is about whether it is better than what already exists.
None of these phases is inherently “better” than the others. The right one depends on you: your diagnosis, your treatment history, your goals, and your tolerance for uncertainty. Talk to your care team, ask questions, and do not assume that clinical trials are only for people who have no other options. They are a part of medicine that is moving faster than ever, and the right trial at the right time could make a real difference.