A placebo is a sham or simulated medical intervention. Sometimes patients given a placebo treatment will have a perceived or actual improvement in a medical condition, a phenomenon commonly called the placebo effect. In medical research, placebos are given as control treatments and depend on the use of measured deception. Common placebos are inert tablets, sham surgery, and other procedures based on false information. Since the publication of Henry K. Beecher’s ‘The Powerful Placebo’ in 1955 the phenomenon has been considered to have clinically important effects.

The word ‘placebo,’ Latin for ‘I will please,’ dates back to a Latin translation of the Bible. In 1785 it was defined as a ‘commonplace method or medicine’ and in 1811 it was defined as ‘any medicine adapted more to please than to benefit the patient,’ sometimes with a derogatory implication but not with the implication of no effect. Placebos were widespread in medicine until the 20th century, and they were sometimes endorsed as necessary deceptions.

The phenomenon of an inert substance resulting in a patient’s medical improvement is called the placebo effect. The phenomenon is related to the perception and expectation which the patient has; if the substance is viewed as helpful, it can heal, but if it is viewed as harmful, it can cause negative effects, which is known as the nocebo effect. The basic mechanisms of placebo effects have been investigated since 1978, when it was found that the opioid antagonist naloxone could block placebo painkillers, suggesting that endogenous opioids are involved.

Placebos exert an ‘expectancy’ effect whereby an inert substance which is believed to be a drug has effects similar to the actual drug. Placebos can act similarly through classical conditioning, where a placebo and an actual stimulus are used simultaneously until the placebo is associated with the effect from the actual stimulus.

Both conditioning and expectations play a role in placebo effect, and make different kinds of contribution. Conditioning has a longer lasting effect, and can affect earlier stages of information processing. The expectancy effect can be enhanced through factors such as the enthusiasm of the doctor, differences in size and color of placebo pills, or the use of other interventions such as injections. In one study, the response to a placebo increased from 44% to 62% when the doctor treated them with ‘warmth, attention, and confidence.

Because the placebo effect is based upon expectations and conditioning, the effect disappears if the patient is told that their expectations are unrealistic, or that the placebo intervention is ineffective. A conditioned pain reduction can be totally removed when its existence is explained.

A placebo described as a muscle relaxant will cause muscle relaxation and if described as the opposite, muscle tension. A placebo presented as a stimulant will have this effect on heart rhythm, and blood pressure, but when administered as a depressant, the opposite effect. Alcohol placebos can cause intoxication and sensorimotor impairment. Perceived ergogenic aids can increase endurance, speed and weight-lifting ability, leading to the question of whether placebos should be allowed in sport competition. Perceived allergens which are not truly allergenic can cause allergies. Interventions such as psychotherapy can have placebo effects.

The placebo effect can work selectively. If an analgesic placebo cream is applied on one hand, it will reduce pain only in that hand and not elsewhere on the body If a person is given a placebo under one name, and they respond, they will respond in the same way on a later occasion to that placebo under that name but not if under another.

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