Types of clinical trials: treatment, prevention, screening

What is a clinical trial?

A clinical trial is a research study that deliberately gives people an intervention – a drug, device, vaccine, procedure or programme – to find out whether it is safe and whether it works^[1]. This is what separates a clinical trial from an observational study, which only watches and measures what happens to people without changing their care^[2].

Clinical trials are also called interventional studies because they involve an active intervention tested in living people, unlike laboratory research, which does not directly involve human participants. Every drug, device, diagnostic test and technique used in medicine today was tested this way before it reached routine care^[2].

Trials are grouped by what question they are trying to answer. The main categories are treatment, prevention, diagnostic and screening trials, alongside supportive care, health services research and basic science studies^[3]. Some sources also describe healthy volunteer studies, biospecimen studies and surveys as related types of clinical research, though these do not always involve testing an intervention directly on your health^[2]. A related concept, clinical research more broadly, also covers research that evaluates aspects of a disease, such as its symptoms and risk factors, in addition to studies of therapeutic drugs or devices^[4].

What is a treatment trial and what happens if you join one?

A treatment trial tests the safety and effectiveness of a new treatment, which may be a medication, a surgical procedure, a new way of combining existing treatments, or a new device^[2]. This is the type of trial most people picture when they hear the words "clinical trial."^[1]

Before any treatment trial reaches people, the treatment is studied in a laboratory. It is often tested in animals first^[2]. If those preclinical tests suggest the treatment is promising, researchers write a trial protocol. It sets out the trial's purpose, the treatments being compared and the outcomes they plan to measure^[1].

Many treatment trials are controlled trials: one group, the intervention group, receives the new treatment, while a control group receives the standard treatment, no treatment, or a placebo if no standard treatment exists. A placebo is designed to look identical to the treatment being tested but has no active effect. This lets researchers see whether any improvement is really caused by the treatment rather than by chance or expectation^[5].

If you join a treatment trial you can expect more exams and tests than in ordinary care, because the research team needs to track your progress closely^[6]. You will go through an informed consent process first: a detailed conversation and written document explaining the trial's goals, procedures, risks, potential benefits, expected duration, and follow-up requirements. It will also confirm that your participation is voluntary and can be stopped at any time^[1].

The consent document will also state whether the trial is randomised, meaning that which treatment you receive is decided by chance, similar to flipping a coin. This means you may not receive the new treatment even if you hope to^[1].

What is a prevention trial and who is usually invited to take part?

A prevention trial studies interventions such as vaccines, medications, and lifestyle practices and habits that researchers think may affect a person's risk of developing a disease. These studies look for the best ways to prevent an illness from starting, or to prevent it from returning after treatment^[2].

Prevention trials often enrol people who are at high risk of developing the condition being studied, rather than people who already have it^[2]. A broader description of this trial type frames it as assessing ways to prevent a disease or condition through medicines, vaccines, and lifestyle changes^[3].

Because prevention trials are testing something meant to stop a disease before it happens, they typically compare the new intervention against no intervention, a placebo, or an existing preventive measure. This follows the same principles of controlled comparison that apply to treatment trials^[5]. Such studies can run for long periods, sometimes many years, since researchers need enough time to observe whether the rate of disease actually falls in the group receiving the preventive intervention compared with the control group^[2].

What is a screening trial and how does it differ from a diagnostic trial?

A screening trial looks for the most effective way to detect, diagnose and monitor a disease, often before any symptoms appear. A well-known example is research into mammography, which found that women of a certain age who had regular mammograms were less likely to die of breast cancer^[2].

Screening trials matter because they help develop ways of catching a disease or condition at its earliest stage, when it is usually most treatable^[2].

A related but distinct category is the diagnostic trial, which evaluates one or more diagnostic interventions meant to identify or confirm a particular disease or condition in someone who may already have symptoms. In practice, screening asks "who has an early or hidden form of this condition within a general population." Diagnosis, by contrast, asks "does this specific person, who already has signs or symptoms, have this condition."^[3]

What other goals can a clinical trial have?

Beyond treatment, prevention, screening and diagnosis, clinical trials can also explore supportive care procedures designed to improve comfort and quality of life for people with a chronic illness. Other categories include health services research and basic science studies, which look at how care is delivered or at fundamental biological questions rather than testing a specific treatment on you directly^[3].

A narrative review aimed at health professionals groups clinical research questions into six categories: treatment, diagnosis, prognosis, etiology, screening, and prevention. Each requires a different study design and each starts from a specific clinical question. That same review stresses that the study design must be matched to the underlying clinical question – a mismatch between question and design is one of the most common weaknesses found when clinical research is evaluated critically^[7]. As a patient, the category that matters most to you is usually the one that determines what will actually be done to you or asked of you during the study.

How do randomisation, blinding and placebo control fit into these trial types?

These three design features can appear in treatment, prevention or, less commonly, screening trials, and they exist to make sure results reflect the intervention rather than chance or expectation^[5].

• Randomisation – participants are allocated to treatment groups at random, usually by a computer programme, so that each group has a similar mix of ages, sex and health status, and any difference in outcome is more likely to be due to the intervention itself^[5];
• Blinding – in a blind trial, participants do not know which group they are in, because knowing might influence how they feel or report their symptoms^[5];
• Double-blinding – neither participants nor the doctors treating them know who is receiving which treatment, which also stops the doctors' expectations from influencing the results^[5];
• Placebo control – the control group is given an inactive substance made to look like the real treatment, so researchers can separate a genuine treatment effect from an effect caused by expectation alone^[5].

If you join a randomised trial, the informed consent document will state this clearly. It will also explain that you may receive the standard treatment, a placebo, or the new intervention. You should not assume in advance which group you will be placed in^[1].

How do treatment trials move through phases, and where do you fit in?

Treatment trials for a new drug typically move through four phases, each answering a different question and each usually involving more participants than the last^[6].

Phase	Main question	Who typically takes part	What is measured
Phase I	Is the treatment safe, and what is a safe dose?	Small numbers of people, often healthy volunteers[8]	Safety, best method of giving the treatment, possible side effects[6]
Phase II	Does the treatment work, and what are the side effects?	A larger group, usually people with the condition being studied[8]	Effectiveness for the specific condition[6]
Phase III	Is the new treatment more effective than existing treatments?	Even larger numbers of participants[8]	Comparison against standard treatment or placebo, effects of different doses and combinations across populations[6]
Phase IV	Is the treatment safe and effective over the long term?	People who take the treatment after it has reached the market[6]	Long-term side effects and effectiveness not identified in earlier phases[6]

People who take part usually do it in Phase III or Phase IV of the trial. Your eligibility for a specific phase, though, depends on your health status and the severity of your condition^[6].

What happens before any clinical trial can start recruiting patients?

Before researchers can enrol anyone, a clinical trial protocol must be developed, setting out the trial's purpose, the treatments under evaluation and the trial endpoints – the outcomes the researchers expect to measure when the study ends^[1].

An ethics committee or institutional review board must review and approve the trial, with its focus specifically on participant safety and participant rights. Only after this approval can the research team begin looking for participants, often through referral from a treating healthcare provider or by direct registration^[1].

What should you ask before agreeing to take part in a trial?

Whichever type of trial you are considering, the informed consent conversation is where you should raise your questions. You are entitled to take your time and ask as many as you need before deciding^[1]. Useful questions include:

• What type of trial is this – treatment, prevention, screening or diagnostic – and what will actually be done to me?^[3]
• Is the trial randomised, and could I receive a placebo or the standard treatment instead of the new intervention?^[1]
• Is the trial blinded, and will I or my doctor know which treatment I am receiving?^[5]
• What phase is this trial in, and what has already been learned in earlier phases?^[6]
• What extra tests, visits or follow-up will be required of me compared with my usual care?^[6]
• Can I leave the trial at any time, and what happens to my care if I do?^[1]

When should you talk to your doctor or care team about a trial?

Speak with your doctor before agreeing to take part in any trial. Speak with them again at any point afterwards if something about the study feels unclear or your health changes during participation. Your well-being remains the trial team's priority throughout, and you retain the right to leave the study at any time for any reason^[1].

For a patient with a serious illness who has run out of effective standard treatments, a trial could be especially useful, since it may sometimes offer access to a treatment not yet available to the general population. Discuss with your care team how any costs, such as tests and visits related to the trial, will be handled^[6].

Summary

Clinical trials are not a single kind of study. A treatment trial tests whether a new drug, device or procedure works and is safe; a prevention trial tests whether something can stop a disease from developing in the first place; a screening trial looks for better ways to detect a condition before symptoms appear; and a diagnostic trial evaluates a way of confirming a condition once signs or symptoms are already present^[3].

Whichever type of trial you are invited to join, the same protections apply: a written protocol, review by an ethics committee, and an informed consent process that explains the purpose, the risks, whether the study is randomised or blinded, and your right to withdraw at any time^[1]. Ask your care team which type of trial you are being offered before you decide.

1. [1] What Is a Clinical Trial? (accessed 2 June 2026) – https://my.clevelandclinic.org/health/articles/clinical-trial
2. [2] Clinical Research: What Is It? (accessed 2 June 2026) – https://www.hopkinsmedicine.org/research/understanding-clinical-trials/clinical-research-what-is-it
3. [3] How do clinical trials work? (accessed 2 June 2026) – https://www.medicalnewstoday.com/articles/278779
4. [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC10023071/
5. [5] Understanding Clinical Trials (accessed 2 June 2026) – https://www.chelwest.nhs.uk/your-visit/patient-leaflets/links/copy_of_UnderstandingClinicalTrialsNorthWestLondonBooklet20200217.pdf
6. [6] Clinical Trials: A Guide for Patients (accessed 2 June 2026) – https://www.webmd.com/a-to-z-guides/clincial-trial-guide-patients
7. [7] Clinical research methods for treatment, diagnosis, prognosis, etiology, screening, and prevention: A narrative review (accessed 2 June 2026) – https://pubmed.ncbi.nlm.nih.gov/32445266/
8. [8] Clinical Trials (accessed 2 June 2026) – https://www.hopkinsmedicine.org/clinical-trials