Clinical trials are conducted in stages, known as phases, to answer different research questions as a new treatment or intervention is developed.
• Phase 1 trials focus on safety. They test a new treatment in a small number of people to understand how it works in the body and identify any side effects.
• Phase 2 trials look at how well the treatment works and continue to monitor safety in a larger group of participants.
• Phase 3 trials compare the new treatment to current standard treatments in a larger population to confirm effectiveness and monitor side effects.
• Phase 4 trials take place after a treatment has been approved and is in general use. They collect longer‑term information about safety and benefits.
There are also different types of clinical trials, including treatment trials, prevention trials, observational studies, and medical device studies. The specific type and phase of any trial you are considering will be clearly explained to you in plain language before you decide whether to take part.