In clinical trials, a placebo may be given to one group of participants instead of the active drug or treatment in order to assess whether the latter is actually effective.
A sugar pill containing no medication or a sham treatment given to a patient in order to give them the expectation that they will get well.
In controlled clinical trials, one group may be given a real medication or treatment while another group is given a placebo. The purpose of using the placebo is to discern if observed effects are due to the treatment or to the power of suggestion or other non-treatment related factors.
The Placebo Effects
The word placebo is Latin for "I will please," and the placebo effect has been known for centuries as the ability of some treatments with no medical value to have a positive effect on a person's health so long as they believe that the treatment does have value.
Traditionally, placebos have been sugar pills and given to people in place of real medications. This is done, for example, in double-blind clinical trials of new medicines to see if the medications do any better than placebos. If the placebos produce improvement for 30% of patients and the medication produces improvement in 31% of patients, then it probably does not do anything significant. Very often, the anecdotal reports of the benefits of various pseudoscientific alternative medicines can be attributed to the placebo effect.
How does the placebo effect work? Some argue that the mind itself has healing powers - thus, when you think you are getting a medicine, your mind works even harder to help you get better. Others argue that there is no real "placebo effect" and that what we perceive is simply the natural healing process taking its course. Recent studies demonstrating an observable and measurable effect in the brain on those who receive placebos, but which is different from those who receive real medication, indicates that whatever role it this plays may be small.
There may also be, curiously enough, a nocebo effect - the creation of pain, illness and more because of suggestion. Research on this is scanty, but suggestive. One study caused asthma attacks in asthmatic patients after they were given nebulized saltwater which they though was an irritant. They got better after they were treated with the same saltwater, this time told that it was a medicine. In Japan, boys who reported having severe allergic reactions to lacquer trees were blindfolded and told they were being rubbed with leaves from a chestnut tree (when they were actually being rubbed with leaves from a laquer tree) and vice-versa. Most of those who thought they got the chestnut leaves had no reaction. Those who thought they got laquer leaves had a severe allergic reaction.