What are the phases of clinical trials?

Clinical trials are an essential part of the process which determines whether a new drug, medical intervention or device is suitable, safe and efficient to use on patients.

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Thousands of clinical trials take place each year, and they all follow a particular sequence of stages, called phases. Most clinical trials only follow phases 1 to 3, although in certain cases a former (phase 0) and later (phase 4) stage is also required.

Below is a brief outline of how each phase works and what it aims to achieve:

Phase 0

Phase zero trials only involve a small number of people and are used to discover whether a particular drug works in the way it is expected to. The developers of the drug will be looking to see what effect it has on the patient – for example, if a cancer drug reaches the cancer.

Only small amounts of the drug are used during this stage, so it is unlikely to offer any real benefit to the patients involved, but they are also less likely to experience any side-effects.

Phase 1

Relatively small, phase 1 trials involve testing the new drug or device in a controlled manner, with close monitoring of the patients to see what effects take place.

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According to the NHS, patients in phase 1 are often healthy individuals. Any side-effects are carefully noted, and if a drug is deemed effective, higher doses may be given.

These types of adaptive phase 1 clinical studies – such as those offered by – are used to determine variables, such as the correct dosage required for different patients.

Phase 2

If successful enough, a phase 1 trial may be stepped up to the larger phase 2, where up to 100 patients may be involved.

These stages escalate the testing, to discover more about the drug’s potential, and any side-effects and how these can be managed. They often involve comparison with placebo groups as well as randomised grouping.

Phase 3

If a new drug makes it to phase 3, it will then be directly compared against the best treatment currently available. Thousands of patients can be involved.

Phase 4

If a drug has been proven to work and consequently licensed, it may still be trialled through phase 4. This involves monitoring long-term risks, benefits and side-effects in thousands of patients.

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