Not always, and in many trials, not at all. Plenty of studies compare a new treatment against an existing one rather than against a placebo. Where a placebo is used, you'll be told the chance of receiving it before you consent, and in many patient trials the placebo is given on top of standard care rather than instead of it.
The placebo question is one of the top reasons people hesitate to join a trial, so it deserves a straight answer rather than a reassuring one. Here it is in full.
What a placebo is, and why trials use one
A placebo is an inactive version of the study treatment: a tablet, injection or device with no active ingredient, made to look identical to the real thing. Trials use them because people often improve simply from being treated, monitored and expected to improve. To know whether a treatment works, researchers need to separate its effect from that response, and a placebo group is how they do it.
Without that comparison, a treatment that does nothing can look like it works. Placebo control is what protects future patients from being prescribed it.
When trials don't use placebos
A placebo isn't the default, and using one isn't always allowed. Under the Declaration of Helsinki, withholding an effective treatment purely to run a placebo comparison is generally not acceptable. In practice:
- Active comparator trials test the new treatment against the best existing one, so everyone receives an active treatment
- Add-on designs give every participant standard care, and randomise only the extra: standard care plus the new treatment versus standard care plus placebo. Nobody goes untreated
- Open-label trials have no placebo at all; everyone knowingly receives the study treatment
- Placebo-only designs are mostly limited to conditions where no proven treatment exists, or where briefly pausing treatment is medically safe and the ethics committee has accepted the justification
So the real question isn't "will I get a placebo" but "what is the control group in this trial getting". Ask it in exactly those words.
How do I know my chances of getting a placebo?
The randomisation ratio is stated in the Participant Information and Consent Form. A 1:1 trial gives you a one-in-two chance of each group. A 2:1 trial means two participants receive the treatment for every one who receives placebo, which is common precisely because it improves your odds of active treatment. If the ratio isn't in the form, ask, and don't consent until the answer is specific.
Allocation itself is done at random by a computer once you've passed screening. Nobody at the site chooses your group, and nobody can move you between groups on request. That's the point of it. Randomisation happens after consent, which is why it pays to know your rights before you sign rather than after.
Blinding: who knows which group you're in?
In a double-blind trial, neither you nor your study doctor knows your group. This isn't secrecy for its own sake: knowing changes behaviour, and changed behaviour distorts results in both directions.
Two things stay true throughout. First, the site can unblind you in an emergency; if you need urgent care and the treating team must know what you've been taking, there's a rapid process for exactly that. Second, blinding never reduces your monitoring. The placebo group attends the same visits and gets the same safety checks as everyone else.
What happens if I'm in the placebo group?
You receive the same schedule of visits, tests and clinical attention as the treatment group, and in an add-on design you continue your standard care throughout. What you don't receive is the investigational treatment, and that's the honest trade-off to weigh before joining rather than after: it belongs in the same column as the other risks and benefits.
Some trials offer an open-label extension at the end, where participants, including those who received placebo, can access the active treatment. It exists in some studies and not others, so ask at consent rather than assuming.
Can I find out which group I was in?
Usually yes, after the trial ends and the data is locked, though the timing varies from months to longer. Ask two questions at consent: will I be told which group I was in, and when. Getting that commitment upfront is far easier than chasing it later.
If you leave the study early, the same applies: withdrawing doesn't usually entitle you to immediate unblinding, except where your ongoing medical care requires it.
Questions to ask before you consent
- Does this trial use a placebo at all, or an active comparator?
- What is the randomisation ratio?
- Do I stay on my standard treatment either way?
- Under what circumstances would I be unblinded?
- Is there an open-label extension, and who qualifies for it?
- Will I be told my group when the study ends?
A study coordinator should answer all six without hesitation. Where the placebo question fits in the wider journey, from screening to randomisation, is covered in what to expect at each stage.
Sources
- World Medical Association, Declaration of Helsinki (provisions on placebo use).
- National Health and Medical Research Council (NHMRC), National Statement on Ethical Conduct in Human Research.
Related reading: What to expect at each stage · Benefits and risks · The four trial phases explained