Un placebo es una sustancia farmacológicamente inerte que se utiliza como control en un ensayo clínico. El placebo es capaz de provocar un efecto positivo a ciertos individuos enfermos, si estos no saben que están recibiendo una sustancia inerte (ej. agua, azúcar) y que creen que es un medicamento. Esto se denomina efecto placebo y es debido a causas psicológicas.

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A placebo (/pləˈsiːboʊ/ plə-SEE-boh; Latin placēbō, "I shall please" from placeō, "I please") is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient. A person given such an ineffectual treatment will often have a perceived or actual improvement in their condition, a phenomenon commonly called the placebo effect or placebo response. Several different elements contribute to the effect, and the methods of placebo administration may be as important as the administration itself. Placebos are an important methodological tool in medical research. Common placebos include inert tablets (like sugar pills), vehicle infusions, sham surgery, and other procedures based on false information. However, in a 2010 study, patients who knew they were receiving a placebo pill showed better improvement in condition than those who did not receive any treatment. It has also been shown that use of therapies about which patients are unaware is less effective than using ones that patients are informed about. Placebo effects are the subject of scientific research aiming to understand underlying neurobiological mechanisms of action in pain relief, immunosuppression, Parkinson's disease and depression. Brain imaging techniques done by Emeran Mayer, Johanna Jarco and Matt Lieberman showed that placebo can have real, measurable effects on physiological changes in the brain. Placebos can produce some objective physiological changes, such as changes in heart rate, blood pressure, and chemical activity in the brain, in cases involving pain, depression, anxiety, fatigue, and some symptoms of Parkinson’s. In other cases, like asthma, the effect is purely subjective, when the patient reports improvement despite no objective change in the underlying condition. 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. 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. The United Kingdom Parliamentary Committee on Science and Technology has stated that: "...prescribing placebos... usually relies on some degree of patient deception" and "prescribing pure placebos is bad medicine. Their effect is unreliable and unpredictable and cannot form the sole basis of any treatment on the NHS." In 1955, Henry K. Beecher proposed that placebos could have clinically important effects. This view was notably challenged when, in 2001, a systematic review of clinical trials concluded that there was no evidence of clinically important effects, except perhaps in the treatment of pain and continuous subjective outcomes. The article received a flurry of criticism, but the authors later published a Cochrane review with similar conclusions (updated as of 2010). Most studies have attributed the difference from baseline until the end of the trial to a placebo effect, but the reviewers examined studies which had both placebo and untreated groups in order to distinguish the placebo effect from the natural progression of the disease.

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