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The purpose of clinical trials

Clinical trials may be used to:

  • test new treatments/interventions to see if they are better than current treatments or standard care to improve survival;
  • find treatments with fewer side effects;
  • test new combinations of treatments;
  • find treatments or interventions to improve symptoms or quality of life; and/or
  • aid in disease prevention, screening and improving diagnosis.

Pre-clinical studies

Pre-clinical studies include animal studies to explore:

  • drug safety;
  • pharmacokinetics (the study of what the body does to the drug and the processes of absorption, distribution, metabolism and excretion); and
  • pharmacodynamics (the study of the effects a drug has on the body).

Phase 0 trials

These optional trials are very small exploratory studies, using subtherapeutic doses of a novel drug to provide preliminary pharmacokinetic and pharmacodynamic information. These are also known as microdosing studies and typically include less than 15 participants.

Phase I trials

Phase I trials are often the first time a drug is studied in humans. They aim to find out what dose of the drug is safe, what side effects are seen and may provide some initial information on whether the intervention is effective. While patients may derive benefit from them, this is not their main intent.

Dose escalation may be carried out, where the dose of the drug is cautiously increased depending on the side effects experienced. Dose limited toxicities are defined prior to the trial and the maximum tolerated dose can be determined. This information is used to identify a recommended phase II dose (RP2D) for future trials.

Phase I trials typically include 20–50 participants and phase I trials of novel cancer therapies are generally conducted in patients with an advanced cancer diagnosis.

Phase II trials

Phase II trials are larger clinical studies designed to further test safety as well as further exploration of optimal dose, dose frequency and administration routes.

There is a preliminary evaluation of efficacy to decide whether this intervention is worth testing in a larger scale phase III trial. These trials often include around 100 patients.

Phase II trials of anti-cancer drugs are generally designed to recruit patients with a tumour type of particular relevance to the drug under investigation.

Phase III trials

If previous studies have demonstrated safety and potential efficacy, a drug or intervention may progress to a phase III trial.

These are larger trials are designed to establish efficacy compared to the existing standard of care or placebo. If positive results are achieved, these trials can be used for regulatory approval.

These trials generally involve thousands of patients with a particular type and stage of cancer.

Examples where phase III trials may be carried out include in early-stage breast cancer to look at potentially curative treatments, or in advanced metastatic prostate cancer to explore palliative therapies.

Types of phase III trials include:

  • Randomised control trials (RCT): Where participants are randomly assigned to different trial arms. Randomisation ensures groups are matched as similarly as possible for certain characteristics such as age, sex, co-morbidities and disease factors, helping to reduce bias.
  • Blinded trials: Where the participants are not informed of which treatment they are receiving. Double-blind trials also take place, where neither the researchers nor participants know which treatment is being given, helping to reduce bias.
  • Placebo-controlled trials: If there is an existing standard treatment for the condition being investigated, this is typically used as the comparison for the new treatment. If there is no current treatment for the condition, however, then the new intervention can be compared to a placebo: a substance with no therapeutic benefit. This allows any differences between the groups to be attributed to the active treatment.

Phase IV trials

Once a drug or intervention has been approved, further trials will take place to identify less common adverse events and evaluate cost effectiveness, as well as efficacy in populations that may differ from the original study population.

Benefits and disadvantages of participating in clinical trials


Participating in a clinical trial may enable a patient to:

  • receive a beneficial treatment that is not otherwise available to them, and that may be more effective than the current standard of care;
  • feel reassured, as they are closely monitored during the trial; or
  • feel fulfilled, by helping the advance of cancer research for future patients, even if they may not benefit personally.

Potential disadvantages

Despite safety procedures being in place, there are disadvantages and risks associated with clinical trials, such as:

  • a chance that the new treatment could cause harm and not be beneficial;
  • time-consuming additional tests and appointments;
  • financial implications of travel costs and time off work to attend additional appointments, which could be further away at specialist centres;
  • having to meet strict eligibility criteria; patients may undergo additional investigations to find out that they are not eligible for the trial; or
  • additional invasive procedures, such as biopsies, that would not be required outside of a clinical trial.

Key messages

  • There are five phases of clinical trials: zero through to four. Phases zero to three are carried out on the path to approval for use clinically, while phase four occurs after clinical approval.
  • Participating in a clinical trial can give access to a treatment that is not otherwise available.
  • Treatments in a clinical trial up to phase three carry more risk of harm or that the treatment is not beneficial. This is risk is lower once a drug has reached phase four.


For clinicians

For patients

Tagged: Therapies, Cancer

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  • Last reviewed: 09/05/2022
  • Next review due: 09/05/2024
  • Authors: Dr Nicola Campbell, Dr Ellen Copson
  • Reviewers: Dr Terri McVeigh