Placebo

A placebo is a dummy treatment with no active ingredient for the condition under study, such as a sugar pill, a saltwater injection or a sham procedure, made to look and be given exactly like the real thing. Its main use is in clinical trials. In a randomized, placebo-controlled trial, volunteers are assigned by chance to receive either the experimental drug or a placebo, and the outcomes of the two groups are compared. Because symptoms can improve with time, attention and expectation alone, only the difference between the groups shows what the drug itself does.

Such trials are usually double-blind: neither the participants nor the researchers know who got which, so hopes and beliefs cannot tilt the results. The improvement people feel after taking a placebo is called the placebo effect. It is real and measurable, especially for pain, nausea and mood, and appears to involve expectation, learned associations and the release of the brain's own pain-relieving chemicals, such as endorphins. In the opposite nocebo effect, negative expectations produce unpleasant side effects from an inactive treatment.

Giving a placebo when an effective treatment already exists raises ethical problems, so new drugs are often tested against the current standard treatment instead. The word is Latin for 'I shall please'.