The Placebo Effect is when a medical patient is given a "placebo" or fake medicine - one in which there is no ingredient known to have any effect on their stated medical condition, but the patient, believing that the medicine is real, starts to feel better or reports an improvement in their symptoms.
The placebo effect is a common problem in clinical trials of new drugs. In clinical trials, the group who is being tested with a new drug is split into two groups. The first group is given the drug. The second group is given something that has the same appearance as the drug but does not contain the active ingredient.
It is quite common in clinical trials for people in the "placebo group" to report improvements in symptoms.
In order for the drug to be deemed effective in treating a condition there has to be a statistically significant difference between the group receiving the drug and the group receiving the placebo.
The placebo effect produces a psychosomatic response. When a person thinks they are receiving a drug that may help them, they feel better, even if the drug has no clinical effect.
The placebo effect can also extend through the doctors administering the drugs. In order to make trials more accurate a "double blind" clinical trial may be performed - in which both the patient and the physician are "blind" to who received the real drug and who received the placebo.