What is a clinical trial and why is it needed?

What is a clinical trial?

A clinical trial is a research study that involves human participants and is designed to discover or confirm the clinical and pharmacological effects of a treatment, identify its adverse reactions, or track how it is absorbed, distributed, broken down and removed by the body^[1]. In everyday terms, a clinical trial compares the effects of one treatment against another. It may involve patients, healthy volunteers, or both^[2].

The term "treatment" in this context covers a wide range of approaches, not only drugs. Vaccines, surgery, radiotherapy, physical and psychological therapies, educational programmes and methods of diagnosing disease are all tested through clinical trials^[3]. Clinical research as a whole is broader still. It includes both clinical trials, which test an intervention directly, and observational studies, which monitor people in their usual settings without assigning them a treatment. In an observational study, researchers watch what happens to people over time and record outcomes, rather than deciding who receives which treatment^[4].

Why are clinical trials needed?

Every medicine has to undergo a clinical trial in order to be registered and made available to patients. The goal is to test whether a substance is effective and safe for patients with a particular disease, and whether it works better than treatments already in use^[1].

Clinical trials contribute to improving the efficacy and safety of diagnostic and therapeutic approaches used by physicians in daily practice, while advancing medical knowledge and science more broadly. Their results are also taken into account when decisions are made about whether a medicine is reimbursed^[1]. Clinical trials also contribute to public health improvements, help reduce healthcare costs, uncover new understanding of disease biology, and support the development of therapies for rare conditions. These benefits reach beyond any single trial. Findings from one study often inform the design of the next, gradually building a body of evidence that guides treatment guidelines, funding decisions and future research priorities^[5].

Before any drug is approved for use, it must undergo rigorous testing to determine its benefits and possible side effects. This process is necessary to identify potential risks and to determine the appropriate dosage. Clinical trials also let researchers compare a new treatment against existing ones to see whether it is more effective or has fewer side effects. Without this staged testing, there would be no reliable way to know in advance whether a new therapy helps patients more than it harms them. This is precisely the uncertainty a clinical trial is built to resolve. A treatment that looks promising in early laboratory or animal research can still fail once it is tested in real patients, either because it does not work as hoped or because it causes side effects that were not previously seen^[5].

What are the phases of a clinical trial?

Clinical trials of new medicines go through a series of phases to test whether they are safe and whether they work^[2]. Phase 1, 2 and 3 trials are conducted before a medicinal product is authorised and allowed onto the market. Phase 4 trials are conducted afterwards, as needed^[6]. Each phase answers a different question and typically involves a larger number of participants than the one before it, so that a medicine only reaches a wide population of patients once its safety and effectiveness have already been demonstrated at a smaller scale^[2].

Phase	Who takes part	Main purpose	Typical scale and duration
Phase 1	A small number of people, who may be healthy volunteers	First time the drug is tested in humans; researchers test for side effects and calculate the right dose, starting small and increasing only if side effects are absent or minor[2]	Small group
Phase 2	A larger group of people who are ill	Get a better idea of the treatment's effects in the short term[2]	Larger than Phase 1
Phase 3	Larger groups of people who are ill	Tested against an existing treatment or a placebo to see if it works better in practice and to detect important side effects[2]	Often a year or more; several thousand patients[2]
Phase 4	People using the medicine after it is authorised	Safety, side effects and effectiveness continue to be studied in real-world use; not required for every medicine[2]	Post-authorisation, as needed[6]

What happens in Phase 1?

In a Phase 1 trial, the drug is tested in human volunteers for the first time. Researchers focus on identifying side effects and working out what dose might be right for later use in treatment. Doses start small and are increased only if participants experience no side effects, or only minor ones. Because so little is yet known about how the substance behaves in people, this phase is deliberately cautious, with close monitoring at every step before any dose is raised^[2].

What happens in Phase 2 and Phase 3?

Phase 2 moves the medicine into a larger group of people who actually have the illness, to get a better sense of its short-term effects. Phase 3 is only carried out on medicines that have already passed Phases 1 and 2. It compares the medicine against an existing treatment or a placebo in much larger groups, often over a year or more^[2].

What happens in Phase 4?

Phase 4 trials are only carried out on medicines that have passed all previous phases and have already been given a marketing authorisation, meaning the medicine is available on prescription. They continue to study safety, side effects and effectiveness while the medicine is used in everyday practice^[2].

What do control groups, randomisation and blinding mean for a participant?

If you take part in a clinical trial, you will usually be randomly assigned either to a treatment group, which receives the treatment being assessed, or to a control group, which receives an existing standard treatment or a placebo if no proven standard treatment exists. Researchers try to keep every other condition between the two groups as similar as possible. That way, any difference observed can be attributed to the treatment itself^[2].

• Controlled trial – compares a new treatment against a standard treatment, or against a placebo where no standard treatment exists, by setting up a trial group and a control group^[3].
• Placebo – an inactive treatment made to look like the real one; comparing responses to the placebo and to the tested treatment lets researchers tell whether the treatment produces a real benefit^[3].
• Randomisation – participants are allocated to groups at random, usually by a computer programme, so that each group has a similar mix of ages, sexes and states of health, and the treatment groups can be fairly compared^[3].
• Blind and double-blind trials – in a blind trial participants are not told which group they are in; in a double-blind trial neither participants nor treating doctors know, which prevents expectations from influencing how symptoms are felt or reported^[3].

Taken together, these design features exist to rule out alternative explanations for a result. If a trial were not randomised, not controlled and not blinded, it would be far harder to know whether an improvement came from the treatment itself or from some other factor, such as the natural course of the illness or the participants' own expectations^[3].

What ethical and legal protections apply to trial participants?

Clinical trial procedures are strictly regulated. The ethical and legal conditions that apply are designed to protect participants' rights and safety. Quality assurance measures conducted in accordance with Good Clinical Practice (GCP) ensure that these ethical and legal conditions are followed and that the trial results are reliable^[6].

A trial must be justified by results from earlier preclinical or clinical data, scientifically justified and described in a clinical trial protocol, based on ethical principles, and conducted by people with appropriately high professional qualifications, at a research centre^[1]. Trials must also comply with the ethical principles set out in the Declaration of Helsinki, alongside Good Clinical Practice. For example, a clinical trial for a vaccine or biomedicine to be conducted in Germany must first receive a positive opinion from the responsible ethics committee before it can proceed. That opinion is taken into account by the national competent authority. This means that no trial can begin on the basis of a sponsor's or researcher's judgement alone; an independent body must first weigh the anticipated risks against the potential benefits for participants^[6].

All health and care research also has to meet high standards of ethical approval before people can take part^[4]. Extensive information on clinical trials conducted in the EU is published in databases accessible to the public, such as the EudraCT database, which primarily covers past clinical trials, and the Clinical Trials Information System (CTIS)^[6]. Trials recruiting in Europe can also be searched on clinicaltrials.eu. Prospective registration of a clinical trial is considered necessary for transparent research, and helps reduce publication bias and selective reporting of results^[7].

What is the difference between commercial and non-commercial trials?

A commercial clinical trial is conducted to obtain a marketing authorisation for a medicinal product or medical device, or to amend an existing authorisation. Pharmaceutical or biotechnology companies act as sponsors of these trials. A non-commercial clinical trial is conducted to advance clinical practice and to search for new treatments. The distinction matters mainly for who funds and initiates the research, rather than for the standards the trial has to meet. Both commercial and non-commercial trials are subject to the same ethical review, protocol requirements and Good Clinical Practice obligations. In practice, this means a patient enrolled in a non-commercial, academically led trial can expect the same rigour and the same safeguards as one enrolled in a large industry-sponsored study^[1].

What types of clinical trials exist?

Different clinical trials are designed to answer different research questions, and each type helps researchers learn something specific that can help people in the future^[8].

• Treatment trials – involve people who already have a condition and test new treatments, or new ways of using existing treatments^[8].
• Prevention trials – usually involve people who do not have the condition but are at high risk of developing it, or who have had it and are at high risk of a recurrence, and look at ways to prevent it^[8].
• Screening trials – test ways to find cancer before it causes symptoms, when it may be easier to treat^[8].
• Supportive care / palliative care trials – look at ways to improve quality of life for people with a condition, particularly those experiencing side effects from the disease or its treatment^[8].

Each of these trial types sits within the same overall system of phases, controls and ethical oversight described above; what changes is the population studied and the specific question the study is trying to answer. A person considering a trial invitation may therefore find it useful to ask which of these categories the study falls into, since that alone reveals a great deal about who is eligible and what the study is trying to find out^[8].

What does taking part in a clinical trial actually involve?

Taking part can be time consuming. You may be expected to attend a number of screening and follow-up sessions, and some trials require an overnight stay. There may also be restrictions on what you can and cannot do – for example, you may be asked not to eat or not to drink alcohol for a period of time. You may experience unknown side effects from the treatment being tested^[2].

Some clinical trials offer payment, which can vary depending on what is involved and expected of you. Others do not offer payment and only cover travel expenses. It is worth finding out about the inconvenience and risks involved before signing up, and weighing up carefully whether it is worth it for you^[2].

Taking part in clinical research is always a personal decision. Whether or not you decide to take part, you will still receive the best treatment available for your condition. Knowing in advance that your standard of care will not be affected by your decision can make it easier to weigh the practical demands of a trial against its possible benefits, without feeling pressured either way^[4].

Why do people join clinical trials?

People join clinical trials for many different reasons. People living with a condition such as cancer often join because they want to help future patients. People with certain risk factors want to help doctors learn how to prevent the disease, and healthy volunteers want to help doctors learn how to detect it early^[8]. If you take part, you may be one of the first people to benefit from a new treatment. There is also a chance the new treatment turns out to be no better, or worse, than the standard one^[2]. Some people take part to have access to new, experimental treatments that are not otherwise widely available^[4]. These motivations often overlap, so a single participant may be balancing a personal hope for benefit against a broader wish to contribute to knowledge that will help other patients later on^[8].

When should you talk to a doctor or a patient organisation before joining a trial?

Reliable and complete information about clinical trials is especially important early in the treatment process, so that you can make informed decisions about how your treatment path should proceed. Clinical trials are sometimes treated as a treatment of last resort, which can make it harder for patients to weigh up the risks and benefits properly. Raising the topic with your doctor as early as possible is worthwhile^[1].

You can ask your doctor or a patient organisation whether they know of any clinical trials you may be eligible to join. You can also search dedicated registers or contact researchers yourself^[2]. Before agreeing to take part, it is worth asking about what the trial will involve, what the known and unknown risks are, and what happens if you decide to withdraw. Asking these questions before signing anything, rather than after, gives you the chance to compare a trial against your other treatment options while there is still time to change your mind^[3].

Summary

A clinical trial is a structured, regulated way of finding out whether a treatment works and is safe, moving step by step through phases that grow in scale and scrutiny before a medicine reaches everyday practice^[2]. The safeguards built into this process – ethics committee approval, informed consent, randomisation, blinding, and public registration – exist specifically to protect the people who take part^[6].

For a patient or carer weighing up an invitation to join a trial, the decision remains personal and never affects the standard of care you would otherwise receive^[4]. Understanding the phase a trial is in, what group you might be assigned to, and what is expected of you day to day gives you the grounds to ask the right questions before you decide^[3].

1. [1] Patient in Clinical Trials – Information folder (accessed 2 June 2026) – https://abm.gov.pl/download/1/8525/Informationfolder-updatedversion30.pdf
2. [2] NHS – Clinical trials (accessed 2 June 2026) – https://www.nhs.uk/tests-and-treatments/clinical-trials/
3. [3] Chelsea and Westminster Hospital NHS Foundation Trust – Understanding Clinical Trials (accessed 2 June 2026) – https://www.chelwest.nhs.uk/your-visit/patient-leaflets/links/copy_of_UnderstandingClinicalTrialsNorthWestLondonBooklet20200217.pdf
4. [4] NNUH Research – What is research? (accessed 2 June 2026) – https://nnuhresearch.nnuh.nhs.uk/what-is-research/
5. [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC11721763/
6. [6] Paul-Ehrlich-Institut – Clinical Trials (accessed 2 June 2026) – https://www.pei.de/EN/regulation/clinical-trials/clinical-trials-content.html
7. [7] https://pmc.ncbi.nlm.nih.gov/articles/PMC3877534/
8. [8] National Cancer Institute – What Are Clinical Trials? (accessed 2 June 2026) – https://www.cancer.gov/research/participate/clinical-trials/what-are-clinical-trials