What Is a Blinded Clinical Trial?

A blinded trial is a study where you do not know which treatment you are getting. This sounds strange at first. But it exists for a good reason. It helps researchers get an honest answer about whether a treatment actually works.

In a single-blind trial, you do not know which treatment you are getting. Your doctor does know. In a double-blind trial, neither you nor your doctor knows. A third party, usually a study statistician, holds that information until the trial ends. Double-blind trials are considered the strongest design, since neither side can, even without meaning to, treat one group differently.

If you knew you were getting the new drug, you might expect to feel better. That expectation alone can change how you report your symptoms. Doctors can be affected the same way. They might watch a patient more closely, or interpret a vague symptom differently, if they know which treatment that patient is getting. Blinding removes this kind of bias. It helps make sure the results reflect the treatment itself, not anyone's expectations about it.

Blinding does not mean you are kept in the dark about the trial itself. Before you join, the informed consent process explains the trial's purpose, the possible treatments involved, and how blinding works in that specific study. You will know what the possible treatments are. You just will not know which one you personally received, until the trial reaches a certain point or ends.

Many people worry that a blinded cancer trial means they might get a placebo, a fake treatment with no real drug in it, instead of any treatment at all. In cancer treatment trials, this is uncommon. Most cancer trials compare a new treatment against the current standard treatment, not against nothing. When a placebo is used, you are told about that possibility in advance, as part of informed consent.

Yes. Every trial has safety monitoring built in. If a data and safety monitoring board sees a serious safety problem, or a clear result in one direction, they can unblind the trial early or stop it. You can also usually ask to be unblinded for a specific medical reason, like an emergency, at any point.

Blinding almost always goes hand in hand with randomization. Randomization means a computer, not you or your doctor, assigns you to a treatment group by chance. This happens before blinding kicks in. Together, randomization and blinding are the two biggest tools trials use to get a fair, unbiased answer to the question being studied.

Not every cancer trial can be blinded. If one treatment is a pill and the other is an infusion given at a clinic, it is hard to hide which one you are getting. Surgery trials are almost never blinded, for the same reason. In these cases, researchers use other tools instead, like having an independent reviewer, who does not know your treatment group, assess your scans or test results.

Not knowing which treatment you are getting can feel uncomfortable, especially when you are already dealing with a cancer diagnosis. This is a completely normal reaction. Remind yourself that both options in a blinded cancer trial are almost always active, reasonable treatments, not one real treatment against nothing. You are not gambling with your care; you are helping answer a question that will help patients like you in the future, while getting real treatment yourself.

Ask whether the trial is single-blind or double-blind. Ask what the possible treatments are, even though you will not know which one you get. Ask whether a placebo could be involved. Ask what happens, and how quickly, if there is a safety concern that requires unblinding.

• FDA — Patient-Friendly Language for Cancer Clinical Trials.
• National Cancer Institute — How Clinical Trials Work.