A placebo is a simulated or a medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient. Sometimes patients given a placebo treatment will have a perceived or actual improvement in a medical condition, a phenomenon commonly called the placebo effect. Placebo effect consists of several different effects woven together, and the methods of placebo administration may be as important as the administration itself.

In medical research, placebos are given as control treatments and depend on the use of measured suggestion. Common placebos include inert tablets, vehicle infusions, sham surgery, and other procedures based on false information. However, placebos may also have positive effect on a patient's subjective experience who knows that the given treatment is without any active drug, as compared with a control group who knowingly did not get a placebo.

Placebo effects are subject of recent scientific research aiming to understand underlying neurobiological mechanisms of action in pain relief, immunosupression, Parkinson disease and depression. Brain imaging techniques showed that placebo can have real, measurable effect on physiological changes in the brain. The objective physiological changes have been reported, from changes in heart rate and blood pressure to chemical activity in the brain, in cases involving pain, depression, anxiety, fatigue, and some symptoms of Parkinson’s, but in other cases, like asthma, the effect is purely subjective, when the patient reports improvement despite no objective change in the underlying condition. Placebos are widely used in medical research and medicine, and the placebo effect is a pervasive phenomenon; in fact, it is part of the response to any active medical intervention.

The placebo effect points to the importance of perception and the brain's role in physical health. However, the use of placebos as treatment in clinical medicine (as opposed to laboratory research) is ethically problematic as it introduces deception and dishonesty into the doctor-patient relationship.

Placebo effects can last for a long time: over 8 weeks for panic disorder, 6 months for angina pectoris, and two and half years for rheumatoid arthritis. Placebo effects after verbal suggestion for mild pain can be robust and still exist after being repeated ten times even if they have no actual pharmacological pain killing action.

Similar to the placebo effect, inert substances have the potential to cause negative effects via the "nocebo effect" (Latin nocebo = "I will harm"). In this effect, giving an inert substance has negative consequences.

Another negative consequence is that placebos can cause side-effects associated with real treatment. One example of this is with those that have already taken an opiate, can then show respiratory depression when given it again in the form of a placebo.

Withdrawal symptoms can also occur after placebo treatment. This was found, for example, after the discontinuation of the Women's Health Initiative study of hormone replacement therapy for menopause. Women had been on placebo for an average of 5.7 years. Moderate or severe withdrawal symptoms were reported by 40.5% of those on placebo compared to 63.3% of those on hormone replacement.

Placebos do not work for everyone. Henry K. Beecher, in a paper in 1955, suggested placebo effects occurred in about 35% of people. However, the response rate is wide, ranging from 0% up to nearly everyone. In a dental postoperative pain model, placebo analgesia occurred in 39%. In research upon ischemic arm pain, placebo analgesia was found in 27%. The placebo analgesia rate for cutaneous healing of left hand skin was 56%.

In social anxiety disorder (SAD) an inherited variant of the gene for tryptophan hydroxylase 2 (enzyme that synthesizes the neurotransmitter serotonin) is linked to reduced amygdala activity and greater susceptibility to the placebo effect.

The placebo effect occurs more strongly in some conditions than others. Dylan Evans has suggested that placebos work most strongly upon conditions such as pain, swelling, stomach ulcers, depression, and anxiety that have been linked with activation of the acute-phase response.

In the opposite effect, a patient who disbelieves in a treatment may experience a worsening of symptoms. This effect, now called by analogy nocebo (Latin nocebo = "I shall harm") can be measured in the same way as the placebo effect, e.g., when members of a control group receiving an inert substance report a worsening of symptoms. The recipients of the inert substance may nullify the placebo effect intended by simply having a negative attitude towards the effectiveness of the substance prescribed, which often leads to a nocebo effect, which is not caused by the substance, but due to other factors, such as the patient's mentality towards his or her ability to get well, or even purely coincidental worsening of symptoms.

Placebos used in clinical trials have sometimes had unintended consequences. A report in the Annals of Internal Medicine that looked at details from 150 clinical trials found that certain placebos used in the trials affected the results. For example, one study on cholesterol-lowering drugs used olive oil and corn oil in the placebo pills. However, according to the report, this "may lead to an understatement of drug benefit: The monounsaturated and polyunsaturated fatty acids of these 'placebos,' and their antioxidant and anti-inflammatory effects, can reduce lipid levels and heart disease." Another example researchers reported in the study was a clinical trial of a new therapy for cancer patients suffering from anorexia. The placebo that was used included lactose. However, since cancer patients typically face a higher risk of lactose intolerance, the placebo pill might actually have caused unintended side-effects that made the experimental drug look better in comparison.

See also:
https://en.wikipedia.org/wiki/Placebo
http://www.physicsdemos.juliantrubin.com/encyclopedia/psychology/placebo.html

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)