Merck Manual

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Shalini S. Lynch

, PharmD, University of California San Francisco School of Pharmacy

Last full review/revision Jul 2019| Content last modified Jul 2019
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Placebos are substances that are made to resemble drugs but do not contain an active drug.

(See also Overview of Drugs.)

A placebo is made to look exactly like a real drug but is made of an inactive substance, such as a starch or sugar. Placebos are now used only in research studies (see The Science of Medicine).

Despite there being no active ingredients, some people who take a placebo feel better. Some others develop "side effects." This phenomenon, called the placebo effect, appears to occur for two reasons. The first reason is coincidental change. Many medical conditions and symptoms come and go without treatment, so a person taking a placebo may just coincidentally feel better or worse. When this change occurs, the placebo may incorrectly be credited with or blamed for the result. The second reason is anticipation (sometimes called suggestibility). Anticipating that a drug will work often actually makes people feel better.

The placebo effect is mainly on symptoms rather than the actual disease. For example, a placebo will never make a broken bone heal faster, but it may make the pain seem less. Some people seem more susceptible to the placebo effect than others. People who have a positive opinion of drugs, doctors, nurses, and hospitals are more likely to respond favorably to placebos than are people who have a negative opinion.

When a new drug is being developed, investigators conduct studies to compare the effect of the drug with that of a placebo because any drug can have a placebo effect, unrelated to its action. The true drug effect must be distinguished from a placebo effect. Typically, half the study's participants are given the drug, and half are given an identical-looking placebo. Ideally, neither the participants nor the investigators know who received the drug and who received the placebo (this type of study is called a double-blind study).

When the study is completed, all changes observed in participants taking the active drug are compared with those in participants taking the placebo. The drug must perform significantly better than the placebo to justify its use. In some studies, as many as 50% of participants taking the placebo improve (an example of the placebo effect), making it difficult to show the effectiveness of the drug being tested.

Placebo: I Shall Please

In Latin, placebo means “I shall please.” In 1785, the word placebo first appeared in a medical dictionary as “a commonplace method or medicine.” Two editions later, the placebo had become “a make-believe medicine,” allegedly inactive and harmless. Now, the profound effects of placebos, both good and bad, are well known.

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