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Let's put a fictitious case, that of patient X. Several times a day, for several days, it causes pain, which is controlled by a dose of morphine. Until the last day of the experiment. Those 24 hours, without Mr. X knowing, the morphine is replaced by a saline solution absolutely innocuous. It seems incredible, but this solution has the same effect as morphine and the pain disappears.
It is what is known as the placebo effect. Before the arrival of drugs in the twentieth century, it was the most potent weapon in medicine against the disease. Crocodile excrement, worm oil, lizard blood and even being touched by the King were medicines used between the sixteenth and nineteenth centuries. Since the publication, in 1955, of the book The Powerful Placebo by H.K. Beecher, it was recognized that 35% of patients with a wide variety of diseases could be treated only with placebo. In subsequent studies, it has been seen that it can work in 70% and even 100% of cases.

No one yet knows what mechanisms are involved in the placebo effect. Some studies on pain suggest that it reduces anxiety and facilitates the release of endorphins (natural chemicals similar to narcotics) in the brain, although they are hypotheses not yet confirmed