Types of clinical trials

Clinical trials are research studies involving people. They assess whether a medical intervention is safe, effective or better than existing care. An intervention might be a medicine, vaccine, medical device, surgical procedure, diagnostic test or a change in how care is delivered.

Trials can be grouped in several ways: by their purpose, the stage of testing, or how they are designed. These categories can overlap. For example, a trial may test a new medicine in Phase III using a randomised, controlled design.

Types of trial by purpose

Treatment trials

Treatment trials test new or existing approaches to treating a condition. They may compare a medicine with a placebo, with standard treatment, or with another intervention. They can also investigate combinations of treatments, different doses or new ways of delivering care.

Prevention trials

Prevention trials explore ways to reduce the chance of developing a disease or health condition. They may study vaccines, medicines, lifestyle changes or other approaches. Some involve people at increased risk of a condition, while others involve people from the general population.

Diagnostic trials

Diagnostic trials assess tests or methods used to identify a disease or condition. Researchers may examine how accurately a test detects a condition, whether it can identify it earlier, or whether using it helps guide treatment.

Screening trials

Screening trials investigate ways of finding a condition before symptoms appear. They may compare a new screening test with the usual approach, or assess whether screening improves health outcomes.

Supportive and quality-of-life trials

These trials look at ways to improve comfort, daily functioning and wellbeing for people living with an illness. They may study pain relief, psychological support, rehabilitation, nutrition or ways to manage treatment side effects.

Health services and care-delivery trials

Some trials investigate how healthcare is organised or delivered. For example, they may assess a new follow-up service, a digital health tool or a different approach to coordinating care.

Phases of medicine trials

Trials of medicines are often described by phase. Each phase has a different main purpose, although some studies combine phases. The number of participants and the level of monitoring vary between trials.

Phase I

Phase I trials are usually the first studies of a medicine in people. They mainly assess safety, side effects and how the medicine behaves in the body. They often involve a small number of participants. In some areas, including cancer research, early trials may involve people who have the condition rather than healthy volunteers.

Phase II

Phase II trials provide early evidence about whether a medicine may work for a particular condition. They also continue to assess safety and help researchers understand which dose may be appropriate. These trials usually involve more participants than Phase I trials.

Phase III

Phase III trials test a treatment in larger groups, often comparing it with standard care or a placebo where appropriate. They can provide stronger evidence about benefits and risks and may support decisions about whether a treatment should be approved for use.

Phase IV

Phase IV studies take place after a medicine has been authorised and is in use. They can help identify less common or longer-term side effects, assess how well the treatment works in routine care, and compare it with other available treatments.

Not every treatment follows exactly the same path. The phases may be adapted for different types of intervention, and a trial’s phase does not by itself tell you whether it is suitable for a particular person.

Common clinical trial designs

Randomised controlled trials

In a randomised controlled trial, participants are assigned by chance to different groups. One group may receive the intervention being studied, while another receives standard care, a placebo or a different intervention. Random allocation helps reduce the influence of factors that could otherwise affect the results.

Blinded trials

In a blinded trial, some people involved do not know which intervention a participant has received. In a single-blind trial, participants may not know; in a double-blind trial, participants and relevant study staff may not know. Blinding can reduce the risk that expectations influence how outcomes are reported or assessed. It is not possible or appropriate in every trial.

Placebo-controlled trials

A placebo is an inactive treatment designed to resemble the treatment being tested. A placebo group can help researchers distinguish the effect of an intervention from other influences. Placebos are used only when it is ethically acceptable; participants should not be denied effective treatment where doing so would put them at unreasonable risk.

Active-comparator trials

These trials compare a new intervention with an existing treatment rather than with a placebo. They can help researchers find out whether the new approach is more effective, similarly effective, safer or easier to use.

Crossover trials

In a crossover trial, participants receive more than one intervention in a planned sequence, with a break between treatments where needed. Each participant can act as their own comparison. This design is suitable only for certain conditions and treatments, because the effects of one intervention must not carry over and distort the results of another.

Parallel-group trials

In a parallel-group trial, participants are assigned to one group and usually remain in that group for the study. This is a common design for comparing treatments.

Cluster-randomised trials

Rather than assigning individuals, these trials assign groups—such as GP practices, hospitals, schools or communities—to different approaches. They are useful when an intervention is delivered to a whole group or when individual allocation would be impractical.

Pragmatic trials

Pragmatic trials assess how an intervention works in everyday healthcare, often across a broad range of patients and settings. They can help show whether results are likely to apply in routine practice.

Clinical trials and observational studies

Clinical trials assign participants to an intervention or care approach according to a research plan. Observational studies, by contrast, observe people’s health and care without assigning an intervention. Both types of research can provide valuable evidence, but they answer different questions and have different strengths and limitations.

Taking part in a clinical trial

Every trial has eligibility criteria, which set out who can take part. Before deciding, potential participants should be given clear information about the study’s purpose, what participation involves, possible benefits and risks, and their right to ask questions. Taking part is voluntary, and participants can usually withdraw without giving a reason; this should not affect their usual care.

A clinical trial should be reviewed and authorised as required, and its conduct should be monitored to protect participants. Anyone considering taking part can discuss the study with their healthcare professional and ask the research team about its design, alternatives and any costs or practical commitments.