Trial Phases Explained
Every clinical trial goes through stages, called phases. Here is what each one means for you.
The journey of a new treatment
Phase 1: “Is it safe?”
Who participates?
A small group of people - usually 20 to 100 healthy volunteers.
How long?
Several months
What's involved?
Researchers test the treatment for the first time in people. They want to find out if it is safe and what side effects there might be. They also work out the right dose.
Risks
Because this is the first time the treatment is given to people, there is more uncertainty about side effects. However, it has already been through extensive laboratory testing.
Phase 2: “Does it work?”
Who participates?
A larger group - usually 100 to 300 people who have the condition being studied.
How long?
Several months to 2 years
What's involved?
Now researchers test whether the treatment actually works for the condition. They continue to monitor safety and side effects.
Risks
There is a better understanding of safety from Phase 1, but there may still be unknown side effects. The treatment might not work for everyone.
Phase 3: “Is it better than what we have?”
Who participates?
A large group - usually 1,000 to 5,000+ people with the condition.
How long?
1 to 4 years
What's involved?
This is the big one. Researchers compare the new treatment with the current standard treatment to see which is better. This is usually the last step before a treatment is approved for general use.
Risks
By this stage, the treatment has been studied extensively. Risks are much lower, and any common side effects are well understood.
Phase 4: “What happens long-term?”
Who participates?
Thousands of people who are already using the approved treatment.
How long?
Ongoing - sometimes many years
What's involved?
After a treatment is approved and available on the NHS, researchers continue to monitor it. They look for long-term effects, rare side effects, and how it works in the real world.
Risks
The treatment is already approved and in use. This phase is about gathering extra safety information.
Which phase might be right for me?
If you are healthy and want to help research, you might be suitable for Phase 1 trials. These are often looking for healthy volunteers.
If you have a condition, you are most likely to be offered a Phase 2 or Phase 3 trial. These are testing treatments for your specific condition.
If you are already on a treatment that was recently approved, your doctor might invite you to take part in a Phase 4 study as part of your ongoing care.