Placebo

What is a placebo?

A placebo is an intervention used as a comparison or treatment context without the specific active component under study. A placebo effect is a change caused by aspects of that context, such as expectations or learned associations.

Benedetti and colleagues (2011) review how these processes can affect symptoms and brain activity. Their effects vary by outcome and mechanism; a placebo is not simply a treatment that “does nothing.”

Placebo effect versus placebo response

Improvement in a placebo group, often called the placebo response, is broader than a placebo effect. Natural recovery, changes in reporting and regression toward typical symptom levels can also contribute. Research needs appropriate comparisons to separate these causes.

A trial example

Imagine a randomized trial comparing an active medicine with a matched tablet lacking that medicine’s active ingredient. Both groups receive the same schedule of visits and symptom measurements. If symptoms improve in the placebo group, that change alone does not identify a placebo effect: some participants might have recovered naturally, entered the study during an unusually bad period, or reported symptoms differently over time.

A randomized no-treatment comparison, where appropriate, can help estimate how outcomes differ with and without the placebo treatment context. Other designs may control particular parts of that context. Such comparisons still require care about expectations, attention, reporting, and what outcome is measured. The active-versus-placebo comparison and the placebo-versus-no-treatment comparison answer different questions.