Placebo and control groups in clinical trials explained

What is a placebo, and what is it not?

A placebo is an inactive substance or sham procedure – a sugar pill, a saline injection, or a mock surgical intervention – that has no direct pharmacological action on the condition under study. Some placebos are described as "impure": they contain a substance that is biologically active in the body, but not for the condition being treated in the trial^[1].

A placebo is not the same as giving a participant nothing. Receiving a placebo still means going through the same clinic visits, examinations, and interactions with medical staff as the group receiving the active treatment. It means being weighed. It means being questioned about symptoms. It means being monitored for side effects. It means being given the same amount of attention and reassurance as any other participant in the study. This shared "ritual of treatment" is part of why placebo groups often show measurable improvement even though they received no active drug^[2].

Why does a clinical trial need a control group at all?

A control group is the group against which the experimental treatment is compared; it may receive a placebo, the current standard treatment, or no treatment. The randomised controlled trial (RCT) design is considered the most rigorous method for establishing whether a cause-and-effect relationship exists between a treatment and an outcome^[3].

Random assignment to the experimental or control group is what makes the two groups comparable at the start of the trial. Ideally, the only systematic difference between them is the treatment itself. Without this comparison, it is not possible to tell whether an improvement came from the drug, from natural recovery, or from participants simply being observed and cared for^[3].

A documented example of why this matters: in the ALLHAT trial, the arm using the drug doxazosin was stopped because it was linked to twice as many heart-failure hospitalisations and a quarter more cardiovascular events than the diuretic chlorthalidone. Because this particular comparison had no placebo arm, however, the trial could not determine whether doxazosin was actually harmful or simply less effective than the diuretic. This is precisely the kind of ambiguity that a properly designed control arm is meant to resolve. Its absence in this instance left an open question that active-controlled comparisons alone could not fully answer^[4].

How strong can the placebo effect actually be?

The placebo effect is a genuine psychobiological phenomenon, not simply a sign that "nothing happened" – research shows it can be robust in both laboratory and clinical settings^[5]. In one Veterans Affairs Cooperative Study of patients with stage 1–2 hypertension, 30% of participants on placebo reached a diastolic blood pressure below 90 mm Hg, with control rates as high as 38% among older white participants^[4].

In depression studies, placebos have been reported to be about 75% as effective as antidepressant drugs; in pain studies, they work about 50% as well as active pain relievers^[4]. A study published in Science Translational Medicine on migraine pain medication found that a placebo labelled "placebo" was 50% as effective as the real drug at reducing pain after a migraine attack^[2].

Neurobiological research links some placebo effects, particularly pain relief, to the body's own opioid system. Several studies have shown that placebo analgesia can be partially or completely reversed by the opioid antagonist naloxone, a finding that points to a real biochemical mechanism rather than a purely psychological illusion^[5]. Functional brain imaging shows that opioids and placebos activate the same brain regions, including the cingulate cortex, and that people who respond to placebo show greater activation of brain regions with opioid receptors than non-responders. This overlap in brain activity helps explain why placebo analgesia can feel, physiologically, remarkably similar to relief produced by an actual painkiller^[6].

Placebos affect symptoms that the brain modulates, such as the perception of pain, stress-related insomnia, or nausea and fatigue linked to cancer treatment – they do not lower cholesterol or shrink a tumour^[2].

Why does the placebo response vary between studies and conditions?

• The characteristics of the placebo itself – larger pills are perceived as a stronger dose than smaller ones, and two pills can seem more potent than one; injections tend to produce a stronger placebo effect than pills^[1].
• The participant's own expectation of benefit, which raises the chance of a placebo effect even when the person is sceptical that it will work^[1].
• The relationship between the participant and the treating clinician – greater trust in the clinician is associated with a stronger placebo response^[1].
• Some conditions are self-limiting (such as the common cold) or naturally go through remission (such as multiple sclerosis or lupus), so improvement during a placebo period may simply reflect the natural course of the disease rather than any effect of the placebo^[1].

When is it ethical to use a placebo control, and when is it not?

The Declaration of Helsinki states that the benefits, risks, burdens and effectiveness of a new method should be tested against the best current proven method, and that this does not preclude the use of placebo, or no treatment, in studies where no proven method exists. A strict reading of this principle rules out placebo-controlled trials whenever an authorised, effective therapy already exists for the condition, favouring active-controlled trials instead^[7].

At the same time, the European Agency for the Evaluation of Medicinal Products (the EMA's predecessor body) noted that trials seeking to show that a new drug and an active comparator have similar efficacy are inherently less reliable than trials seeking to show superiority over a comparator, whether that comparator is inactive or active. Where the judicious use of placebo remains essential to demonstrate a new medicine's value, active-controlled trials are still encouraged wherever a methodologically acceptable proof of efficacy and safety can be obtained that way. This tension – between the scientific value of a placebo arm and the ethical duty to offer proven care – is exactly why regulators require a case-by-case justification rather than a single blanket rule. The practical effect of this balancing act is that a trial sponsor cannot simply choose whichever design is cheapest or fastest to run. Regulators expect a documented rationale showing why a placebo arm was necessary, what alternatives were considered, and how patient safety was protected throughout the study^[7].

For ethical reasons, participants in a placebo-controlled trial must be told, before they agree to take part, that they may receive a placebo (a "dummy" treatment) rather than the active drug^[1].

What can a control group actually receive?

Type of control	What the control group receives	Typical use case
Placebo control	An inert substance or sham procedure with no expected therapeutic effect[1]	No proven effective treatment exists yet for the condition[7]
Active control	The best current, already-authorised treatment[7]	An effective standard treatment already exists for the condition[7]
No-treatment control	No intervention at all, only observation[3]	Used within the general category of controlled clinical trials[3]

What actually happens to you if you are assigned to the control arm?

Random assignment ("randomisation") means that a computer-based process, not you or your doctor, decides whether you receive the experimental treatment or the control treatment – this is what makes the experimental and control groups comparable at the start of the study. Being placed in the control arm does not mean being left without care: participants in a placebo group attend the same visits, undergo the same examinations, and are followed according to the same study protocol as participants receiving the active treatment^[3].

Placebos may make you feel better for certain symptoms, but they will not treat the underlying disease – they do not lower cholesterol or shrink a tumour^[2]. If you are in the placebo group in a trial where an effective standard treatment already exists for your condition outside the trial, this is one of the key ethical points the trial's ethics committee will have reviewed before approving the study, guided by the principle that placebo is discouraged when a proven effective treatment is already available^[7].

What happens to the comparison once the blinded period of a trial ends?

Some trials continue after the initial blinded comparison ends, moving into an "open-label extension" in which all participants, including those originally on placebo, receive the active treatment. A reanalysis of published data from two Alzheimer's disease drug trials (lecanemab and donanemab), examined separately, found that cognitive decline was steeper during each drug's open-label extension phase than during the preceding double-blind period, and that the extension-phase decline rates were similar to those seen with placebo during the controlled trials. Because individual participant-level data were not available to the investigators, this comparison relied on simulated participant-level trajectories calibrated to the published trial data, together with direct comparison of the reported annualized decline rates; for donanemab, the manufacturer's own extension publications had compared outcomes with untreated historical registry cohorts rather than a contemporaneous placebo group. The study's authors suggested this pattern is consistent with part of the originally observed treatment benefit being a placebo effect that faded once the blinded comparison ended. The authors argue that a contemporaneous, blinded placebo group offers a more reliable basis for judging a treatment's long-term effect than later comparisons with historical patient registries, though their own conclusions rest on this indirect, simulation-based reanalysis rather than on directly observed individual-level data. Once the trial unblinds and everyone knows, or can guess, which treatment they originally received, the psychological conditions that produced the original placebo response are no longer present in the same way. This is why long-term open-label data must be interpreted with considerable caution. Findings of this kind carry real consequences for patients and clinicians. If part of an apparent drug benefit reflects a fading placebo response rather than a durable pharmacological effect, treatment decisions based on early trial results may need to be revisited as longer-term data become available^[8].

How does blinding protect the reliability of a trial and its participants?

In many placebo-controlled trials, participants are not told whether they are receiving the active treatment or the placebo, which allows researchers to compare how each group responds without the participant's own expectation distorting the result. If both groups show the same reaction – improvement or no improvement – this is taken as evidence that the drug itself was not the cause of any change observed^[2].

Even when participants are explicitly told they are receiving a placebo, a genuine effect can still occur. In a migraine study where one group took a placebo openly labelled "placebo," it still reduced pain by about 50% as much as the real drug, an effect the researchers linked to the simple ritual of taking a pill. This suggests that some part of the placebo response does not depend on deception at all, but on the broader experience of undergoing treatment, being monitored, and expecting relief. This distinction matters for how trials are designed and reported. It also matters for patients weighing whether to enrol, since it means that even a fully transparent, honestly labelled placebo arm is not scientifically inert; it can still produce a measurable clinical signal that researchers must account for when interpreting the results^[2].

When should you talk to your doctor or the study team before joining a placebo-controlled trial?

Before agreeing to take part in a trial that includes a placebo or control arm, ask the study team directly whether an effective standard treatment already exists for your condition outside the trial, since this is one of the central ethical considerations reviewed before a placebo-controlled design is approved^[7]. Ask what happens to your usual care while you are in the control group, and ask whether you will be told, at the end of the trial, which group you were in.

If your symptoms change, worsen, or you experience a new problem while in a trial – regardless of whether you believe you are on the active treatment or the placebo – report it to the study team promptly. How such events are handled and followed up should have been explained to you before you consented to take part^[3].

Summary

A control group is the mechanism that turns a treatment claim into a testable comparison: without it, there is no way to distinguish a drug's real effect from natural recovery, the passage of time, or the simple experience of being cared for in a clinical study^[3]. Placebo controls remain one of the most reliable tools available for this comparison, but their use is weighed carefully against whether an effective treatment already exists for the condition being studied^[7].

If you are considering taking part in a trial that uses a placebo or control arm, you have the right to a clear, upfront explanation of what you may receive, how randomisation and blinding work, and what will happen to your care throughout the study. This information is a required part of the consent process, not an optional extra^[3].

1. [1] Placebo effect - Better Health Channel (accessed 2 June 2026) – https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/placebo-effect
2. [2] The power of the placebo effect - Harvard Health (accessed 2 June 2026) – https://www.health.harvard.edu/newsletter_article/the-power-of-the-placebo-effect
3. [3] Types of Experimental Studies - EUPATI Open Classroom (accessed 2 June 2026) – https://learning.eupati.eu/mod/book/tool/print/index.php?id=665
4. [4] Basile J. Analysis of Recent Papers in Hypertension: Placebo-Associated Effects on Blood Pressure Reduction (accessed 2 June 2026) – https://www.medscape.com/viewarticle/407722_5
5. [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC3601706/
6. [6] Placebos: a psychological and biological perspective - Introduction to Biological Psychology (accessed 2 June 2026) – https://openpress.sussex.ac.uk/introductiontobiologicalpsychology/chapter/title-placebos-a-psychological-and-biological-perspective/
7. [7] EMEA/CPMP Position Statement on the Use of Placebo in Clinical Trials with Regard to the Revised Declaration of Helsinki (accessed 2 June 2026) – https://www.ema.europa.eu/en/documents/position/emea-position-statement-use-placebo-clinical-trials-regard-revised-declaration-helsinki_en.pdf
8. [8] Basile J. Analysis of Recent Papers in Hypertension: Placebo-Associated Effects on Blood Pressure Reduction (accessed 2 June 2026) – https://www.medscape.com/viewarticle/antiamyloids-long-term-cognitive-benefits-questioned-2026a1000thv